boostin

To be honest, I don't think I've had to do it for 5 or 6 years now. Maybe oils have improved?

Any oil in the ecu connectors? Yes it creeps up the wiring all the way to the ecu.

Not grinding. If it starts grinding you've gone too hard. And no, I'm not taking the piss.

Getting hard things into tight places.

I can see this going pear-shaped fast lol. Drive along, 50k or so should be fine. Pop it into neutral, and with the clutch pedal out, gently try putting it into the gear with the issue. Not hard enough to get graunching noises though. Do this a couple of times then just drive as normal. Note that this is quite harsh on the synchro ring, so if it doesn't work in the first few tries, don't continue. A gearbox oil change could be beneficial too.

The pumps pop out of the bottles quite easily. Yes you will lose fluid so do it outside. It's only water so you can refill it after. Put a light smear of Vaseline on the rubber seal to make fitting the pump easy.

Just take it back and let them sort it out. Sounds like a worn synchro ring. Mine does it in 5th, but not bad enough to concern me too much yet!

Check powers and earth at the coil connection. You'll probably trace it back to a bad connection at the main engine loom plug.