Interesting reading the posts on this. I had a similar issue a few years ago with a different IS I owned which was due its 70k service and needed the timing belt changed. Lexus Woodford and other Lexus dealerships in London quoted me over £750 plus VAT which was the service and timing belt, not even the tensioner which was extra. I did a lot of ringing around and Lexus Manchester quoted £525 inc tensioner and VAT. Which worked out considerably cheaper even taking into consideration fuel to drive to Manchester and back. Its worth ringing around. Toyota Romford quoted the same as Lexus Woodford.
On the IS I have now, this was my uncles car from new from Lexus Woodford this is serviced by a Lexus mechanic from Lexus Woodford. I can give you his number if you PM me.