The gas fire cutting out may just have been a thermocouple, which is cheap and easy to replace.
Regarding the pump:
There ought to be shutoff valves either side of it, which won't operate.
Then there will be a pair of great big nuts, one each side of the pump, which need to get un-done.
You'll find these are just a bit bigger than any of your adjustable spanners can open to.
Once you obtain some spanners, you'll find the nuts will be seized solid.
Once you've mopped up the blood, and got the pump off, you'll find the replacement pump is slightly different in dimension, requiring you to lengthen or shorten the pipework. And the 'standard' thread on the new pump will be different to the existing nuts on your pipework, necessitating a 3rd or 4th trip to the plumber's merchants.
Just so you know what to expect