Personally I would just pick them up when in screwfix or the like or order online, would never think of going into a shop just to buy a pipe slice, cant see your idea taking off
It isnt the heat exchanger, you need a proper gas engineer , your burner is resonating at low flame and the gas valve needs set up properly, not for a poke and go neewbie
At the top of the heat exchaanger there is a plastic bayonet fitting with a clear hose coming off, turn the bayonet until water comes out of the tube, be carefull the water may be extremely hot
Check your HW off wire is wired in correctly, it should be wired to HW off at the programmer and satisfied at the cyl stat, these two wires connect to the grey on the mid position valve