until April weren't they contributing just 12% to get a 1/40th pension accrual?
I don't know what tax rate a typical MP is on, but for an example, a person earning £65,000 and paying 40% tax, who has a 12% deduction from salary into his pension scheme, actally loses (£65,000 x 12/100 *0.6=) £4,680 out of his pocket. Tax breaks into pensions are especially valuable to higher earners.
Ignoring inflation for simplicity, this person after 20 years would receive half-pay (£65,000 x 0.5=) £32,500 from retirement until he pegs out. If he lives for, say, 25 years, he would receive £812,500 ignoring inflation and index-linking.
In his 20 years of work, he would have contributed (20 x £4,680=) £93,600 net
OOI, at current annuity rates, to receive an index-linked pension of £32,500 with a 50% widows pension, you would need a pension pot of about a million.
So the MP is given a fund worth ten times what he pays in. Note that I have ignored both growth and the increase due to inflation causing final salary at time of retirement being far greater than the salary on which the contributions were based.