Just a quick update re: low flow rate.
Rather than risking buying a new diaphragm and it not fixing the problem I bought a new pump from Pure Freedom and have fitted it this morning.
The result?
A healthy 1.4 lpm on setting two and all the flow I need without hammering the pump or draining the battery! A big thank you to the engineer I spoke to at Pure Freedom who spent a good 10 minutes explaining the possible causes and solutions. The pump arrived at noon the next day with no fuss.
I'd also like to thank the members of this forum who all helped with advice. As I get more experienced I hope to help others who have difficulties. Water fed is the future and I am working quicker and more efficiently doing work that used to take two people longer trad than I can do WFP.