He said my cold water supply tank was smaller than I had told pumpwise
I would say that’s not really relevant.
and I had taken the supply from the vent pipe at the top of the hot water tank, he said I should have fitted a flange. He would not comment on whether this would cause the pump to leak from the casing. I should also say that the pump does not leak from the shaft seal, only the casing.
I assume you had the take off at the bottom of a 45 degree angle vent? If you just teed into it then that’s definitely a no no but Salamander do say you can tee off the bottom of a 45 degree vent but it’s the least preferred option & not specifically recommended for the RSP; personally I always use an Essex flange.
Aerated water causes cavitaion which can generate extremely high local surface temperatures the effects of which are sometimes difficult to believe if the whole lot is running in water. This could cause distortion of the plastic end casings which, in turn, would cause the O ring seals to let by; but cavitaion is more likely to eat the impellers.
I think they are using it to their advantage which is maybe a bit mean but you haven’t really helped yourself by failing to follow the manufacturer’s instructions & compounded it by giving incorrect information on your warranty card; but I’m unsure if either should have made any difference. You could try grovelling or asking if they will supply you with some new impeller housings, you never know your luck. If not, assemble the pump with a good lashing silicone gasket
http://www.threebond.co.uk/PRODUCTS/TB1215LiquidSiliconeGasket/tabid/266/Default.aspx
& see what happens, you may at least get a while longer out of it. But do at least fit a decent take off flange.