I take the last two points on board and, after running some numbers, realise that the situation is not bad as I had thought from reading
http://www.unison.ie/features/budget2003/taxproperty/stories.php?ca=275&si=887763
i.e. unless the index linking was quite close to the actual rate of inflation of houses, both formulae give roughly the same result and the results don't exponentialy deviate after all (I take back my comment about not needing the degree in maths!!)
Example 1A: Old tax scheme (index linked relief, rate @ 40%)
annual inflation = 3%,
house rise = 5%
2004 Cost of House 300
2014 Sell house 300 * (1.05^10 - 1.03^10) * 40% = 300 * 11.4% = 34.2
2024 Sell house 300 * (1.05^20 - 1.03^20) * 40% = 300 * 33.8% =101.7
Example 2A: New Tax scheme (index linking abolished, rate @ 20%)
annual inflation = 3% (N/A),
house rise = 5%
2004 Cost of House 300
2014 Sell house 300 * (1.05^10 - 1.00^10) * 20% = 300 * 12.6% = 37.7
2024 Sell house 300 * (1.05^20 - 1.00^20) * 20% = 300 * 33.1% = 99.2
Thanks for forcing me to look at this again more closely!