[tebbers]

Not only that but you get significant tax relief on your £40k pension contributions (about £8k I think) so takehome plus pension is about £95k from the £100k initially.

[nly]

No, the £40K sacrifice (the max) would be from gross.

Tax 'relief' (it's a refund despite what anyone says) only applies if you pay from your post-tax income.