Keith, I used to own an XM (and CX's before that) - now have a Vel Satis, and I used to live in Lincoln!
The Vel Satis is a 54 plate 2.2 with loads of whistles and bells. Bought it as a one year old with 12,000 miles on it - just over two years later and it's covered 61,000. No major problems - trip meter failed for no good reason (switched to km's - couldn't be re-set by Renault - complete new unit needed) Oh, the
seal behind the flywheel failed and needed replacing. Apart from that, nothing to mention. Discs and pads replaced on the front after 50,000 miles, rear discs and pads still original. When the warranty expired, I extended the manufacturers warranty to 5 years/100k miles.
Service wise, the Renault main dealers (generally) know as little about the VS as Citroen dealers knew about the XM. Controversial opinion coming up - there's nothing fundementally wrong with either car, it's the quality of care and maintenance provided by dealers mmore interested in selling and servicing family hatchbacks.
Driving wise, it's fine - the 2.2 is reasonably quiet and does 35 - 39 mpg depending how hard I can it and how quickly the M4 moves. On a regular run form Wakefield back to Newbury it does 42 if I stick to 70 ish. Very comfortable, plenty of space, particularly for passengers.
On balance, would I go for the XM or the VS? If the XM is really good, I'd go for it in a heartbeat. There are plenty of VS's around - usually 30 - 35 on Autotrader each week. Not many good, late XM's around -Wolsey House Motors generally have some good stock now the XM centre in Oxford has gone over to C5's. Now, when C6's start to drop.....there's a lunatic choice if ever there was one.
Or you could go get a V10 diesel VW Phaeton.....
As for the Avantime - if they'd imported a diesel into the UK, I reckon I'd have had one like a shot.