If it's the diaphragm, you can get to it easily after removing the pump - or is it just its head?
Probably whole pump, but I've never gone at it that way and I'd have to be stood in front of a boiler to know.
I either remove whole DV from front which is relatively easy, or if plenty of space to the left of the boiler I remove the gas valve for ease of operation.
To just do water section pump out would make it easier, but then you'd have top drain boiler anyway so might aswell remove whole DV 6 of one 1/2 dozen.
When I said above about fibre washers OP I didn't mean the little red ones used for tap connectors, I meant proper boiler ones I get mine from Antares you wouldn't be able to, will have to probably buy the Alpha kit of all seals and washers for that boiler about £27 plus vat, quite expensive.