Good evening William,
Sorry for the delay in responding, firstly it sounds like you have correctly identified its the programmer at fault (not the boiler).
There isnt a direct replacement for the EP2002, so you will need to change the back plate, which means to swap the programmer over you need to terminate the wires on the new back plate. The EP2 is sufficient for your needs, but you can chose any CH and HW programmer, you dont need to stick with a Potterton.
In terms of who can do the work, if its done professionally get somone who is PART P certified as per the IEE regulations, this could be an electrican or a CH plumber. However its not more complex than changing a plug socket, so if you are confident in being able to isolate the supply and terminate the wires, I see no reason why you cant do it yourself.
The back plate is the bracket which screws to the wall, and the terminals from the boiler connect to, the programmer clips on to the bracket and normally has a small screw or clip on the underside to secure it in place, with the screw removed or the latch depressed it should allow the bottom to pull out and upm and then the top lift off the bracket.
If you are replacing the programmer with another EP2002 you can just unclip the old programmer and clip the new one back on to the same back plate, however if you decide to use a different programmer eg EP2, you will need to replace the back plate as well.
Instructions for installing the back plate for the EP2 can be found here.
Please let me know if you have any further follow up questions.