Many thanks everyone for your replies, which I really appreciate. Phil, those prices do sound excellent so I will try to source a good independent and see what's possible. I'm in Basingstoke, so if anyone knows of anyone in my vicinity, do let me know, thanks.
Re the car itself, the mileage is low, but it's certainly not been standing for any length of time because the MOTs and service history all suggest a slow, steady and consistent accumulation of miles over the years. It has a pretty good Lexus Bristol history but latterly was serviced at an independent garage also in Bristol. I'm the third owner. The rust on one corner of the sunroof appears to be a fairly known problem on this model. Lexus Reading, where the car is currently sitting, quoted £385.85 for a new glass, but I'm slightly mystified how they would know what it needs because surely the car needs starting before they can open the roof?
In terms of the other prices, in case you're interested, I've been quoted £1,171.28 for the shock absorber and £491.50 for the tie bar bushes. I've also got two blown n/s windows, which they'd like £1,077.63 to replace. I paid £3,800 for the car as a private sale, which I thought was fair for a very nice example, with lots of history and very low mileage albeit with some obvious things to sort. I've already invested in some new Dunlops for it, which were due to be fitted at my local alloy wheel refurbishers (two birds, one stone) on the morning of the ignition issue!
Graham