Pump removal problem

[TicklyT]

A bit late for the OP, but another technique to free stubborn nuts is two hammers (or one hammer and a similar weight or heavier chunk of metal).

Hold the heavier hammer (or weight) against one flat of the nut as an anvil, and give the opposite flat a tock. Working around all the accessible flats helps to free stubborn nuts with less chance of damaging anything than just hammering on it.

 

[tamz]

If it is the big nut you have cut off just use the big nuts off the new valves and fit them to the old ones.
There are 2 small cut outs on the nuts. Line them up with the valve edge and they come off.

 

[johnscales]

All done. Heating on and hot water back.
I did a google search for 'pump valve 28mm' and Tool Station (new one on me) came up. Went to the web site and did a store search and there it was, in the same trading estate as my local Screwfix. Would you believe it. The guy in the store said they've been there for 2 years but they're hidden away which isn't so bad because I regularly find I have to order stuff in from screwfix.
Thanks to all for your help, particularly those who replied yesterday so quickly after I'd asked. Tamz, I thought about using the old valves but didn't notice that I could line up the nuts as you've explained so hacked off the valves and used the newly bought ones. That would have been a real life saver. Look nice and new and the wrench gripped so much better than the old ones which were fairly rounded off before I even started on them.
CHHEEEEEERRRRRRSSSS All.