I've been in contact with Bestway, they are sending out a new liner as a gesture of good will which is very much appreciated
The advisor I spoke with said it was the non return valve within the inflatable liner for the air bubbles - tube A which as it was failing was causing water to go back into the air pump thus leaking from the bottom of the pump unit, apparently it is a common thing as the valve is rubber and perishes over time, they said that I won't get a warranty with the new liner and it'll likely happen to this one too within the next 24 months as it's a "wear and tear item" and "rubber does that" and this is not usually covered by the warranty. From my point of view this is very wrong of them to have a part that will knowingly fail within 2 years of purchase due to "wear and tear" and it can't be repaired you have to replace the whole liner which is as expensive as buying a new spa! all in all I have had about 6 months use of my hot tub before this happened and not even heavy use, I think this is a major design flaw!