I have a 12yr old Volkswagen golf, was running great till afew months ago, starts perfectly sometimes and other times have u sit holding key turned and foot on accelerator for ages

Have you had the fuel pump checked? Fuel filter?

Just two of many possible causes. Battery also.

Thanks all 3 have been checked, mechanic can''t work out what's wrong with it!

Sounds like you need a new mechanic. Do you smell gas when you try to start it?

Spark plugs? Ignition module?
Also, what is the procedure in the owner's manual for starting the engine? Does it recommend holding accelerator pedal down?

Here is what one VW owner found as a cause of starting problems but don't know many specifics from his post on a VW owner's site.
Golf starting problem due to a faulty ignition switch - VW Forum :: Volkswagen Forum