Can anyone recommend a good solvent, cleaner, degreaser to remove oil and grease from my engine parts as I continue to delve deeper into the workings of my K. I am renovating/refurbishing/doing up my bike after it was left unused by me for 5 years and as this is my first bike total strip down since 1970 I am a bit out of touch with products that are available in the UK market. In 1970 I just used to swill every thing in petrol but that's a bit dodgy I suppose.

Most of those products based on citric acid are a lot better than you would suppose. I used Hein Gericke's own brew - unfortunately no longer available - but another very popular one is Muc Off. Avoid the old-style Gunk; it stinks the same but I suspect the formula has been changed because it ain't nowhere near as good as it used to be.

MucOff is good stuff but dont let it on too long. Coke is good stuff too.

Jeyes Fluid will clean it off, hose with water then apply by brush and hose off after a short while, but do not leave too long on painted surfaces.
Learnt this from a mate's old man who had a bike shop in the 60's.

Try a bottle of Marvel Mystery Oil 

- Used with a scotch pad it will clean oxidation and gunk from aluminum without discoloring or staining it. In fact the metal will 'drink' up the oil, providing it with a nice gleam and real resistance to further grime buildup. 
- Dabbed or sprayed on built-up crud and left overnight will enable you to wipe of years of grease, oil and road muck buildup
- Likewise, left on rust spots overnight it will penetrate beneath the rust and allow you wipe most of it off. This works esp. well with chrome. 
- Rejuvenate dried rubber bits by soaking them in a 80% MMO/20% acetone mix. Give it overnight. I use ziplock bags. 
- Pour in the crankcase and run your engine with it until the next oil change. Do it again. A superb and safe de-crudder for rings, top end and valvetrain components

Cheap, safe and effective.

+1 on Marvel Mystery  Oil.  As I recall, on of the main ingredients is Stoddard solvent.  For cleaning parts with a lot of road dirt my favorite is S-100.

Try a bottle of Marvel Mystery Oil 

With a name like that it's got to be worth trying.