I’d advise anyone with the offer of a contract but no agent to negotiate it for them to head straight for the Society of Authors. Once you have that offer in your hand you can join for a relatively low fee (£90 at the time of writing) and once you’ve joined you can send your contract in for a completely free review. The last time I used this service I got four or five closely-typed pages back from them which advised on several revisions I should request, and gave the reasons for those revisions. If I had paid a solicitor for this service it would have cost me several hundred pounds, which made the cost of joining the Society well worth it.
They run lectures on many aspects of publishing, publish a fantastic magazine which is only available to members, always seem to have on hand appropriate statistics and information about the industry, and are very valuable in all sorts of other subtle ways.