So it popping out of reverse is a nuisance, but I've found that if I turn the car off, then put it in reverse, then start the car, then its very likely that it stays in gear and doesn't pop out.
Either way though, its booked to go into the workshop on Wednesday but will try to check the gearbox oil over the weekend myself.
What sort of labour costs is it to replace a gearbox? Cheapest 1.0 box I can find is £96 on eBay!
(I'm just trying to plan ahead as I don't think the whole box is gone, all the other gears work perfectly).