Rover P6 Club

Can anyone report any experience on overhauling the heater fan unit on a P6. The operation of my windscreen blowers is particularly poor. I could blow harder. Much harder.

Is it a case of the fan motor being a bit girly or worn or could it be the sponge seals on the heater flaps have mummified in the period of 34 years? Or a combination of all these. I do know that the fan does not sound particularly urgent when on the higher speed (or is it just good soundproofing)?

Paul