Vagcom is the way forward. I recently installed mine and here is some info that I found to check if it is actually working: (from: carbasics.co.uk/vw_golf_mk4_cruise_control.htm)
I am in edinburgh so if you are close by, you could use my vagcom.
There are two methods using VAG-COM to verify if your ECU has been successfully activated for cruise control. Firstly, when you open the engine module the engine description will include a letter G if cruise is active, eg 1.8L 5V 110KW TLG V005. Secondly, the outputs from block 6 of the engine module will show a 0 in the RH column if cruise is active, but 255 if it is still disabled. If you need to deactivate cruise you can repeat the login procedure above but enter the code 16167.
With cruise control enabled and all wiring completed, it is possible to view the outputs from the cruise switch with VAG-COM by viewing block 66 of the engine module. This may be useful for diagnosis of a wiring fault once installed.