Ive just changed the battery, but the display it doesent work. the model is casio telememo 30

This is a common complaint with many watches. There is obviously something that needs to be done as part of the battery changing that is not included in the user manuals.

Since most manufacturers do not like users to replace batteries in their watches, I thing the exclude any replacement procedures in the instructions; or am I being cynical?

I think you will have to take it to a Timex dealer to sort it out.