Thanks, TLPower. That saved me a few quid at the dealer
The end can was coming off anyway to get to the rear caliper for a pad change. Finally persuaded it to detach without too many anglo-saxon words :-)
Measuring the old clamp it looks like 6cms, but it is slightly deformed where I removed it.
Measuring the tabs on the end can - a few of these I had to loosen, it measures between 62mm and 65mm. With the clamps costing 3.50 each, I ordered the one that looks right and the next size up.
Thanks again