Glo, If replacing the boiler I would go with a condensor boiler like a 'Gerkros' as they are extremely efficent. Grant are not as good as Firebird and the Firebird Condensor unit is not as good as the Gerkros in my view.
Going back to the initial problem, I can't recall the Deluxe 50 boiler off the top of my head but the newer Firebird SuperQ used the Teddington TBB control stat.
On the Super Q you have a fuse unit which is inside the lower removable cover which has a habit of melting if the contacts are not tight and leaves the boiler without current, Simple to fix really. [However if the pump is fed through the control panel and is running this cannot be the problem] Also the Super Q has a high limit stat that auto resets when it cools down and does not need to be manually reset.
Now the Teddington TBB stats give trouble, as in they can run too hot [sometimes causing the high limit stat to come in]. It's simple to recalibrate them without replacement or removal. On the front cover below the knob is a white plastic cap. Remove this and insert a tiny screwdriver to adjust the screw inside. With the pump off, and the boiler on, the boiler will start to boil over [you'll hear it bubbling]. At this point have the stat at max and adjust the screw until the stat turns the boiler off.
This should rectify the problem
Good luck