I feel for you Philip. It took me what felt like ages to find mine. I too was looking for almost the same car - a sub 50K ideally 2008 on facelift SE-L MM auto not in silver (my last two cars were silver and although I like it I was seriously bored of silver) and I also wanted cream leather.
I was willing to travel up to around 250 miles (from the south coast where I live) and even then I had problems. I ended up buying mine from Lexus Stoke as like you I would prefer to buy from a dealership, although I would still consider privately. The car actually seemed relatively good value, especially compared to dealers nearer me.
I ended up having to compromise slightly on the 2008 facelift part, in the end I bought a late pre facelift (57 plate). I couldn't ignore the value though. I had been looking around for ages and the best deal I could find on the facelift (58 plate) from a dealer was 3K more expensive. 3K just seemed like too much of a premium for what is essentially the same car.
I paid 10K for mine back in March. It's a 57 plate with 43K on the clock, Mercury Grey with Ivory leather, auto and multimedia. I love the car and it's in great shape plus I got the year warranty, major service before it left and a couple of other bits thrown in. The best deal I could find on a facelift at the time was from Lexus Leicester for 13K. I would have considered spending 11-12K but 13 just seemed too much considering the quality/value of the car I bought.
It seems you're looking in the right places so I would just hold out and the right car will show eventually - I know it's just painful waiting!