Thanks to everyone, for your support, just thought I'd let you know the outcome for future refernce.
It turned out, that during the removal of tiles from the wall and subsequent replastering, there was lots of debris and dust that had got into the back of the programmer/controller and prevented some of the contacts from making correctly, hence the pump wasn't getting a signal to start up.
I eventually tracked down a full wiring diagram of my system and the first thing I went to check was the controller, luckily that was where the problem lay.
But your suggestions helped me decide on that route.
and subsequently I didn't have to call on a heating engineer.
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