Join Date: Nov 2013
Location: United Kingdom
County: Vale of Glamorgan
Thanks for all the advice and comments. Here's how I eventually got on...
Both the drain plug and filler plug were non-starters. They are basically made of white chocolate and were just going round off as soon as I applied any force to them.
I ended up pumping new oil in via the reversing lamp switch socket. To do this, I removed the battery and battery tray (plus a few additions bits like the top end of the upper boost pipe for access) to get at the top of the gear box. I then used a siphon pump (available from most motor factors, Amazon or places like Machine Mart) to pump a measured amount of oil into the gearbox via the reversing lamp switch socket. Note you need a long reach 13mm socket in order to remove the nut that holds the battery strap in place.
The reversing lamp switch is the electrical connector on the upper part of the gearbox housing. Unclip the electrical contact (plastic lug on the lower side to split them connector) and then remove the switch itself from the gearbox casing using a spanner (17mm or 19mm - can't remember exactly).
I put 2 litres of Castrol standard gearbox oil in (EP 80W - 90). I chose this amount as I suspected there was still some residual oil in the box and there was no over out of the reversing switch socket. Post top-up, the box is much smoother, especially once warmed up, and the notchy feel has largely gone. However, as has been noted elsewhere on this forum, the gear changes between 1,2,3 are stiffer (although still smooth) when the box is cold.