No personal experience with the concierge staff at Claridge's, but I am told that they provide a high level of service. Friends who have stayed there have asked them for some fairly "impossible" tasks, even by luxury hotel standards. The staff did not disappoint.

A fair bit of warning, though. Even the most connected concierge in the city will have trouble getting you into fifteen. The restaurant works on pretty egalitarian principles and keeps fairly strictly to their policy of first come first serve with regard to reservations. I use Quintessentially as a concierge service in London and fifteen had absolutely no interest in setting up a relationship with them in order to provide their fairly well heeled clients with reservations. I have had friends try through the concierge at the Savoy and the Berkeley, also to no avail.

And now a personal opinion. Don't waste your time. I had never waited for a table anywhere else in the world for as long (two months) as I waited for a table at fifteen. I have also never been to a restaurant that was so over-hyped. The bar upstairs is nice and is said to serve a great breakfast or light lunch. The restaurant downstairs is a farce. Overpriced. The service is lackluster and the staff is definitely under-trained and amateur. From the second series of Jamie's Kitchen (it was on here in the UK recently), it seems like I'm not the only one with these complaints.

I think Jamie Oliver is doing a great thing with regard to training unemployed teenagers and giving the profits to charity. But I'd rather just be asked to write a check to charity than trek all the way to Islington and be over-charged for a meal I could easily get at any "hip" restaurant in town.