Year end accounts only clients....

I have a very small accountancy practice that I set up a year ago. So far I have taken on new clients that need monthly book-keeping plus year end accounts work. I receive a regular monthly payment from these clients that covers everything.

Recently, I've had a glut of enquiries from clients that want to do their own monthly book-keeping, but want me to do their year end accounts. This is fine, but not sure how I factor in checking them doing their own bookkeeping - it could be awful & I then would spend masses of time sorting out their monthly bookkeepign before I can get on with doing their year end accounts. Also, at what point do I issue an engagement letter if, say, their year end is ages away so I won't actually be doing anything for them for ages....?

If the client is doing their own monthly book-keeping, do I need to factor in checking the work that they have done?

Sorry for the daft questions, hoping someone can give quick, sound advice!

Many thanks