I'm having problems with one radiator in the house (colder than it should be) and think the problem lies with the 11 year-old Bulldog TRV. From its age, I assume it's a Gen 1 valve. It's the only Bulldog in the house, so I can't just swap the head with another Bulldog to check. I'm happy to buy a complete valve and just use the new head, but the only Bulldogs for sale are Gen 2 and someone has told me that the Gen 1 and Gen 2 heads aren't interchangeable. Can anyone confirm whether this is correct or not? Otherwise I'll have to change the whole valve, which will be a hassle.