If my employer is paying my notice period salary upfront then do I have a legal right to insist that the employer must contribute to the pension fund for the notice period?
As you allude to this is a legal issue, so falls outside of my remit.
I would suggest that you seek legal advice or contact the Citizens Advice Bureau who will be able to help you.
But to give you a steer I spoke to a friend who is a solicitor specialising in employment law. His view is that:
'it depends on the wording in the contract of employment. If it provides that the employer is obliged to pay basic salary only as a payment in lieu, there is no obligation to include any payment in respect of benefits which the employee would have been entitled to receive during the period for which the payment in lieu covers. If the contract is more ambiguous, s/he may have an argument to state that pension should be paid as well.'