Baxi Solo WM 50/4 RS - No Pilot Light - Help!

[c8jay]

I have a Baxi WM 50/4 RS. System is 7 yrs old.
Once or twice a month the heating won't come on until the hot tap has been ran for a while even though the timer is on for HW & CH and Thermostat is high enough.
Second problem!
It was overheating for a day or two. I reset it. Then it would only operate for about 20-30 seconds. Now, the pilot light wont even stay on at all. Should a new thermocouple probably sort it?

 

[kevplumb]

fully pumped or gravity

 

[c8jay]

Fully pumped. Boiler in utility room, tank in airing cupboard upstairs.

 

[kevplumb]

thermocouple is worth a try
more concerned about the overheating
could be a problem with the three port valve