Thanks for your response. I am running the same query each time and not changing the parameters so don't think parameter sniffing is the problem. This query has been running fine for a year on another server which is less specced (one cpu and 2gb memory, but it is the only application using this particulalr server).

I have rebuilt the indexes but to no avail.

I have tried running the query with query hints on the join (hash, loop and merge hints) and the query returns in a couple of seconds. The optimiser is having a hard time compiling the query, but this only started happening yesterday when the query plan changed.