Hotpoint washer - door not opening (model 800 PLUS 9518)?

Hi, I have an ageing Hotpoint 800 Washing Machine. Recently the door won't open at the end of the cycle. I don't think water is being left in the machine but it seems that some release is not being triggered. Often after waiting a long time the door will eventually open. However the wait is getting very long.

Any ideas?

Thanks