You must make sure you are within the scope of your retainer and that you advise on every area that is within such scope. This is a very high test and the first thing any client will look at if considering a negligence claim against you. It will therefore be routine in most retainers that you need to amend the scope of work set out in the Engagement Letter.
To amend the scope of the retainer, simply email the client with a blind copy to the Clients Team. The email should note that the client has agreed to the changes set out therein, but where you wish to be more formal (e.g. where it has been some months since we last undertook work on the matter), then it is a good idea to use the Refresher Engagement Letter available through Keyed-In. Whichever method is preferred, it should address the following points as a minimum: