Baxi Combi Instant 105HE - hot water problem

My son's boiler is a pain! It's a Baxi Instant 105HE. the CH works fine, but DHW is a problem. If I switch the thing off and back on again the boiler fires up, the tap symbol flashes and the temperature rises to 70. and the boiler cuts out. OK so far. If I then switch on the hot water tap there is warm water for a few seconds, but it then runs cold and the boiler does not fire up. I recently changed the DHW temp sender. Anyony any ideas please?
Thanks in advance

The small DHW diaphragm has probably perished or split.

I had to replace one of those just yesterday!

Tony

Thanks for the speedy response. I will get one tomorrow and give it a try.