Attempted my first infusion today, doing a car boot so maybe a bit ambitious! Got as far as -25 on the vac gauge but couldn't get it to stay, but couldn't hear any more leaks after countless laps around the part.

Virtually all moulds will leak to a greater or lesser extent.  resin won't fill the pinholes as air is being sucked in not resin being pushed out.  Pull a vac on mould and flanges and paint on hotcoat.  Vac should pull hotcoat into any pinholes from the back side of the mould.  If in doubt just envelope, safe every time...... unless your bag has holes in it, but thats another story as most vac bag has micro holes in it that can f**k up your infusion if you aren't vigilant.