The other thing that's worth changing is the mechatronic sleeve, as the seals apparently get brittle and start leaking. There are a couple of companies on eBay who stick up service kits for the 6HP19/6HP21 every so often that come with a genuine sump/filter assembly, mechatronic sleeve and fluid (either ZF or Shell branded) for ~£200; you can also get it from
here for £260.
Once you have the kit, it's just the labour to drain down, drop the sump, replace the sleeve, refit the sump and refill (following the slightly complicated ZF procedure that involves filling, bringing the gearbox to ~35 degrees then checking the level). It's on my list for next year...