Call me cynical but why does this not suprise me !!
For motor insurance, the PO use Budget Insurance (BISL Ltd) to place policies on their behalf. Budget inturn are a broker in their own right who on behalf of the PO will have placed your policy with an Insurer in their own right. Check your certificate and the actual insurer will be stated probably at the bottom.
The problem hear appears to be one of a line of communication. The PO is responsible to pay your £50 as it is their promotion and paid out of the commission they receive from Budget. How they get their comm paid may be monthly or quarterly, I don't know.
Did you pay in full or via an instalment plan. If instalments then I suspect there will be a 4th party involved in the process.
Have you received the full policy ? i.e certificate, policy schedule & motor policy full T&Cs ?
If the latter then all parties have agreed to contract therefore there is no reason why you should not have your £50. The initial contract is with the PO & if they state that your £50 would arrive within 28 days then it is actually down to them. However, check the small print...is it 28 days from the policy issue date or 28 days from the attachment date i.e. when you agreed to incept policy via PO & initial payment made to bind cover ?
Even so, IMO, they are simply stalling.
Fire off a complaint stating policy number etc to
consultation@royalmail.co m
This is the main contact for complaints involving PO products & services.
I hope this has helped in some way. If so, please give my scales a quick click.
car Insurance via Post Office, had your 'cash back'? 
