Hi Mark
Having a look through the 11a, 111 and 109v8 optional parts catalogue, and it does show mud-shield's for the brake drums, so they were available.
Wether you can still gat them is a diferent matter, but it might be worth popping to your local stealer and asking if the part no's are still 'good'.
Series 111, 88" Mudshield Kit - RTC 8061 (From August 1980)
Series 11, 11a, 111 109" Mudshield Kit 528163
This should, all being well, give you a start point.
H T H Chris