So Newbie here is working on a 85 K100RS that was stripped when I brought it. Basically the bike has no front end on it and no bodywork. I have everything connected except the headlight, taillight, heated grips and indicators. Tank is connected. This bike has sat in a shed near the coast for 3 years.

Put a new battery in and nothing. Nothing on the dash and fuel pump not working. There is power to the start relay, all fuses seem to be good and there is power to the ignition key. Have roughed up frame earth but the earth to the gearbox is seized so cannot get it undone. Is this earth important? I have been told it could be one of the relays! Any ideas?

If my memory is correct the earth on the gearbox goes directly to the battery.  On a bike that hasn't had it's wiring altered this earth is the ONLY connection to the negative post of the battery.  So, yes it is important.

+1 that. If its seized its a good indication that its well corroded and not earthing at all.
Another important earth, besides the one you've cleaned on the frame under tank, is the starter motor. make sure the brushes are clean and in spec. Also, because the starter motor earths thru its casing, the bed it sits on (2 small bolts at the back) should be clean as well.

Use the Google search bar on the Forum or Portal tabs to search for key words on the forum. It works well for finding specific info.
Also, go to the Portal page and read You Just Bought a Used K100, What Now and also work thru the trouble shooting procedure on EFI/Electronic Ignition Troubleshooting

Well it must be the earth connection. Have spent many hours trying to get the bolt out. You guessed it stripped it! Now looking at drilling out the bolt. Oh the joy.

As a test just fit another earth to the battery and ground it to the engine near the one you are trying to undo. If it works then you know where you are at. Let us know what happens.

You could go to the wreckers and grab a longer and a little thicker battery cable from a car and hook it up to another bolt, Worth a try at least.

So drilled out the bolt and hooked to the cable to the case where the Alt cover  bolts up. Silence, nothing, zip, not a thing happening. Its go to be a connection somewhere? as mentioned I have everything connected except heated grips, headlight, taillight rear brake light switch and indicators!

The alternator cover is plastic and even tho the bolt goes thru to the casing, you may not be getting a connection because you wouldn't be able to tighten it enough on plastic.
Just attach it to one of the starter fixing bolts. Pretty sure its the same size bolt and the cable should easily reach. Make sure the bed the starter bolts to is clean.

Are you sure the ignition switch is working properly?  You say you have power there, but can't get power to the rest of the bike.

I just had a similar problem with my bike, only it was intermittent.  On mine, even though there was no crud or corrosion in the switch the moveable contact had shallow grooves worn into it that did not make reliable contact.  Repaired the surface, and, knock on wood, problem solved. 

Checking should be easy, just hot wire the switch.  Corrosion, wear, dirt and a crappy design make that switch a weak point.

Ignition switch next port of call. They are known to go wrong with age.

It turns! So I bridged the clutch cut off switch and she turns. Now it won't fire! I have fuel pressure, spark and when I pump the throttle bodies I get a wet plug but no go! I suspect injectors?

The only way to get fuel into your engine is via the injectors. Have you put fresh fuel into it and cleaned the old fuel out.

Pumping the throttle bodies won't do anything. If you have wet plugs then the injectors are probably working ok.
Make sure you have the HT leads connected in the correct order. The coils are labeled 1&4 , 2&3

Follow the troubleshooting procedure.

How wet are the spark plugs?  If the injectors are stuck open they could be flooding the engine. 

If you haven't already, get the old fuel out and get 2 gallons of fresh gas with a good cleaner in there.  Mix the cleaner with the gas before putting it in and run the starter to circulate the fuel for 5-6 seconds.  Do that five or six times and if the engine doesn't start let it sit overnight with a battery charger on it.  Next day try again after the overnight soak.  Check the plugs again to see how wet they are.  

It might take a couple days to get enough varnish out to let the injectors work.  If not, you will have to take them to a shop that cleans fuel injectors. 

It is also possible that the air flow sensor is stuck open and calling for more fuel than the engine can take and causing it to flood.  Can't tell you how to check that, but I am sure there is someone here who can.  Mostly I suspect bad gas or stuck injectors from sitting for three+ years.

I'd agree that bad fuel could be the problem and best to clean it out. As for the injectors, I think it's more likely that they would stick shut than open if they've been left to sit and less likely that all of them would be stuck open imo.
Best to take them out if they're suspect and clean and check them properly. Read up about this first as there is dirt, o-ring, filters etc to consider before removal.

You say you "get a wet plug" but have you checked that all SPs are wet? and that you have spark on each one?

Make sure you don't crank it for too long. You can damage the starter. 5-10 sec at a time is enough. Also keep your battery charge maintained while you go thru this process. 12.5V (70% charge) and above.

Update. Sorry should have said the tank and rail where empty so no fuel at all. I put premium ULP in and tried all of the above mentioned. Anyway a mate of mine is an ex-BMW merchanic and we agree injectors. So have got them out and off for a pro cleaning (ultrasonic). Will put new orings on and put em back in to see what happens.