I recently had the same drama, firstly I would get access to your switch and check its working with a retest light, check continuity from switch output to pump connector, check any earth inside rear guard area and verify its actually earthing ! Have fun
I (as a trade qualified motor mechanic) used to work at 'Autocarbs' where we would acid bath all components of the carburettor, before fitting kits, repairs etc. to me - that was not done Ie: dirty screws etc ? Did they show you old parts/ kit that was replaced ? Mmm