The voltage readings seem ok, so double check the battery connections are clean and tight. Could be a sticky starter motor also, had this a couple of times with my old Mk1 Focus - I took mine off, sprayed a little brake cleaner in it, left it an hour for it to evaporate then squirted a little WD40 in there, was fine after that.
I had a yellow Mk1 with bubble arches, 8" wide white Weller's, 2.1 pinto with twin 40's, quick rack, 5 speed quick shift gearbox, anti dive kit, 5 linked axle with anti !Removed! bars, lowered 2" on bilsteins, Ashley single box exhaust system..... Loads more! Spent a fortune on that thing!
Mate of mine just bought a shell to build up, sooooo jealous!
Great news! But don't you feel better now knowing all the mods are declared so they have very little chance now of saying "Ooh, black wheels Miss LauMk7? That's a change you didn't tell us about - have fun paying for your own repair costs!"
Trust me, some companies will use anything to wriggle out of a claim, so I declare everything no matter how trivial (even the scoobyclinic.com stickers in my Impreza's windows are declared! Give them nowt!