All engine faults/repairs/maintenance reluctant starting (battery alternator)
Hacksawbob If you cant get the engine running by the key you maybe able to jump start it off another vehicle (or canal boat! are they 12V?) or beg borrow steal one of those portable jump start devices)
Once you get the engine running, check the charge at the battery with a voltmeter to see if its getting a good charge, 14V ish, Check the out put from the back of the alternator to see what the difference is(*check voltage between the body of the alternator (-ve) ond one of the wires leaving it), if its bad at the Alt (<13V) then swap out the alt for a known good one or contact your friendly local GSF or get a second hand one, I think I have seen a few on the for sale section, I had one reconditioned for about £50, I think they retail for about £90 exchange. I have one of unknown condition in my shed which you can borrow but postage maybe crippling. If it has got enough juice leaving the back of the Alt but not reaching the battery then wiring needs to be checked, particularly the earths Gearbox-to-chassis and the "crown of earths" behind the fusebox in the glove box (disconnect battery when doing this) Also don't forget to clean the battery terminals with a bit of emery paper and make sure connections are nice and secure, Also check where the -ve earth strap comes off the battery and connects under the battery this needs to be shiny too, as does the usually braided wire that connects between.
This assumes of course you can get the thing started! If not you can only do continuity checks to see if there is a break in the wiring somewhere(beep option on the voltmeter) and do the earth checks and hope for the best.
Possibly the battery of course, take battery out and charge of a 240V battery charger for 24Hrs then leave for another 24 and check voltage drop need to be near to 12V or buy a new one (bosch from GSF about £40?) Or borrow one!
Another candidate is the starter motor main connection it goes from the battery to the Alt VIA the starter motor and it is specifically positioned by VW to catch a fresh mist of lovely oxidising roadspray near your off side rear wheel arch, on a 2WD you can see it from the engine bay follow the thick wire from your Alt it will lead you there. Check for looseness, corrosion, German U boats etc that may impede the Flow of energy. there should be a rubber boot covering this but often not.