Hi Neil
RIP to your father first and foremost; not a nice thing to have to deal with after the passing of a loved one.
unfortunately this is one of the earlier contracts that don't have any 'escape clauses' and the only way the redemption penalty can be avoided is a massive increase in gilt rates (not happening).
Nowadays the equivalent would have a penalty free option at death of one of the mortgage holders, or a downsize option; or simply be at 3% which is more palatable. The earlier loans were pretty strict and one of the reasons equity release has such a bad press.
Saw a client recently who had a similar situation and they were pretty much fooked as the options were pay the penalty or carry on at 8%; as they were elderly they opted to carry on as the odds were if they paid the penalty they might not outlive the break even point.
might be worth speaking to Aviva or going on social media to try and guilt them but according to the letter of the deal I think you are stuck with them.
sorry