Well As any easy way of working it out

Taking the maximum cash amount allowed for a couple £11,280 to invest in an ISA and use average saving interest rates for the last 21 years Taken from here

After 21 years you would have £441,912.59 which is 60k a bit off your £500k but for Zero risk

It is interesting to see how quickly that can mount up with zero risk cash accounts, but £11,280 per year is almost £250k and I guess Son of Taeper will not have had to invest that much in the capital repayment element of his mortgage over the years. £1k per month + interest would have been a hell of a mortgage in 1990!