If you go for T40 Richard then you will only have to oversize by around 35%
Controlling your required store temp and still having a "full" cylinder is a bit of a challenge I suppose., you may be able to use the boiler SP temp as the required store temp. I think you stated in your other thread that you have a 17.6KW Boiler, assuming a boiler circ pump flow of 20LPM then the boiler deltaT is 12.6C, say 13C at full output, so at stat cut out (50C) the store temp is 63C but if the stat hysteresis is say 8C then the hot water to the store is only 55C (42+13) but ideally you are looking for a constant store temp, just thinking out loud, if you set the boiler SP to say 60/65C and set the store stat to 80C how would that work out?, the boiler will then fire as required and depending on the store bottom temp will fire either flat out or modulate (if you have a modulating boiler).
Controlling your required store temp and still having a "full" cylinder is a bit of a challenge I suppose., you may be able to use the boiler SP temp as the required store temp. I think you stated in your other thread that you have a 17.6KW Boiler, assuming a boiler circ pump flow of 20LPM then the boiler deltaT is 12.6C, say 13C at full output, so at stat cut out (50C) the store temp is 63C but if the stat hysteresis is say 8C then the hot water to the store is only 55C (42+13) but ideally you are looking for a constant store temp, just thinking out loud, if you set the boiler SP to say 60/65C and set the store stat to 80C how would that work out?, the boiler will then fire as required and depending on the store bottom temp will fire either flat out or modulate (if you have a modulating boiler).