On further investigation I came across a company who make covers to order and the size I wanted was indeed listed and pictured under 'Shirtmaster'.
I have now ordered (just hope it is the correct one!)
For your information the comapny details are as follows, should you ever be asked the same question by another desperate shirtmaster owner:
Ironingboardcovers,
5 groomfield close,
tooting bec,
london,
uk,
SW17 8DH,
UNITED KINGDOM,
Tel - 020 86720769
www.ironingboardcovers.co.uk