I know it's been a long wait, but I've eventually got round to taking my gear lever out.
The bearings that are in mine now are marked 'HK 1214L'.
These are 18mm OD, 12mm ID and 12mm deep. One pressed in from either side.
Anyway, going through the RS catalogue, I've came across these:
IKO Nippon Thompson bearing ref TLA1212, which is exactly the same size and costs £1.665 + VAT each
I'm going to get a couple of these ordered and should have them fitted before the weekend (unless I've read my vernier wrong in which case I'm stuffed 'cos I had to smash one bearing out to get the OD correct).
I'll let you know what happens.