Welcome to the forum!
Perturbing knocking from under drivers and passenger's feat (suspected anti rollbar bushes - droplinks and wishbones recently replaced)
If you've already done upper & lower wishbones + droplinks, I'd agree that ARB bushes is the next thing to check. You can apparantly change the ARB bushes without droping the subframe. A chap called Pat (Forum name SEARs) had a load of bushes that you could change on the ARB rather than removing the ARB and dropping the subframe. Just to drop and replace the subframe is about 5 hours or so, which obviously is quite alot of money. If you take the wheels off and disconnect the drop links, you should be able to check for play in the bushes. The other thing to consider is engine mounts.
- non-centralising gear stick (have tried lubricating gear linkage to no avail)
If you've already had a go at the linkage, did you do this from the gearbox end or the gear stick end? Sometimes a little plastic bush wears out in the gearstick ball joint which can prevent the stick to self center. If you do a search on google for Alfa Eper it's the electronic parts catalogue and you can see all the part numbers and pictures of the bits. It's hosted in the Netherlands and is 2005, so the prices will have gone up a bit since it was produced.
- Coolant temperature sitting at about 65 degree (sender replaced, suspected thermostat)
I'd say you need a new thermost. They are around £60 - £70.
- Noisy heater fan at settings 3/4
I'm not 100% sure how, but you can take the info display off the center consol and access the fan in there. I think it just gets off balance when all the dust and stuff sticks to it over the years. As I understand it, a quick hoover normally corrects the noise problems. (I've not done it myself, so I may be way off the mark on this one, just something I've read!!)
- small oil leak from oil return from bottom of turbo (knackered gasket/o-ring?)
There is a rubber hose coming out of the turbo back into the sump. Sometimes these split and oil seeps out. There should be a little heat shield over it that can easily be removed and expose the pipe. After a quick clean you should be able to see where the leak is coming from. The pipe is a right pain to replace though as it's alot more ridig that the one on there and it takes alot of wiggling and swearing to get the hose on both pipes(turbo & sump).
- reports of clipping clutch from garage (this was back in April, I haven't felt it slip (I've tried!) but bite is near the top of the pedal)
If it's near the top, it sounds like the friction plate might be worn. I wouldn't replace it until it starts slipping personally. It might be worth bleeding the clutch when you're doing the thermostat as it's in that area. For me, the fluid was black and massively improved the clutch bite after bleeding.
It might help or steer you in the right direction.