> As I am jointly asessed with my wife can
I claim high rate tax relief from both our incomes.
I don't really understand what you mean by the above. If you have paid 42% tax on any income that is jointly assessed then you will get that back on the pension contribution. If the pension contribution exceeds the amount on which you paid 42% tax in the relevant year the you will get 20% tax back on the balance. Does this make sense?
On the other hand if you are asking if you can contribute 30% of your joint earnings to YOUR pension then this is not possible as far as I know as I believe that the 30% is reckoned on individual earnings even for married couples - i.e. you could contribute 30% of your earnings to your pension and she could contribute 30% of HER earnings to her pension.