Are letting agents legally bound to assist in handover to another agent?

Letting Agents

We are in the process of changing letting agents; at the moment they are being co-operative in agreeing to share all necessary documents with our new agents, however, they do not know yet that we have a complaint prepared, and we think they are eager to pass us on because they know they have liability for a problem in billing and a past tenant. We want to use their current goodwill but we don't know whether if we delay sending the complaint, it will affect their willingness to look into it, since we will no longer be clients. On the other hand, if we send the complaint now, it might mean that they will not co-operate with the hand-over. It is worth mentioning that they very quickly gave us two months' notice to find another agent when we complained internally just to the property manager herself; this is highly unusual and therefore suggestive of their knowing they have made some big mistakes. Normally an agent will try to charge between two and four months' rental income if the client wants to change agencies.