Found this cleaning procedure on the MG Owners Club Forum -
MG TD TF 1500 - Cleaning original headlamp reflectors
You have to be very careful with the silvering.
I have had good result with immersing the whole unit in hot water with normal washing up liquid and rinsing it a lot.
Resist the temptation to wipe it with any rough cloth because it scratches immediately.
After cleaning rinse repeatedly with clean water and dry with a hairdryer.
Apparently reflectors can be resilvered, but that will be costly?
Author - Willem vd Veer
Cheers Spike