Seems I have a leak. Suspecting either main seal or the o ring on the output shaft. Have started my reading up on how to go about it. What I'm wondering is; what other issues are likely to crop up during the job? I'm thinking rusted bolts etc, other jobs to do whilst the gearbox is out and stuff like that. Hopefully I'll try and order parts all at the same time.
Cheers. 

I did that job back in February and did everything else while in there.

A few things....

New rear seal....get the BMW one, not the alternatives.
New O ring.
I put in new clutch plate, Sachs is good and they make the BMW ones so don't buy the BMW badge.
Also new clutch bolts, flywheel nut.
Did the gearbox and final drive seals too.
Also replaced driveshaft boot, clutch pushrod boot and their clips, clutch pivot arm bearings [pig to get old ones out].

If you are in that far worth taking out sprag clutch and doing a clean on it.

I knew my alternator monkey nuts were good, but easy enough to get at any time.

New exhaust nuts, studs, gaskets.

WD40 the whole darn lot for ages before hand, anything you need to be opening including the exhaust studs/nuts, . Its so worth it.

While your doing it fit a grease nipple to the clutch arm. Greasing it stops water and road crap getting in and pushes any out.
Regards Martin.

Be careful removing the gearbox on your K75. Unlike the K100 you can't remove the clutch push-rod first and it very easily damaged. Get three or four 8mm studs at least 100mm long to replace the GB bolts as you remove them and slide the GB back clear of the push-rod.

There's a special tool for aligning the clutch when reinstalling. If you have one.... throw it away. Its easier to do it by eye.

After you've removed the swing arm, clean out the pivot bearings and check for any slack or side movement. If they're ok pack them really well with new grease.

Fantastic! Thanks very much for the advice. That'll all be built into my planning!
I'll probably give myself a couple of days so I hopefully won't be to rushed.