When I was swapping the output valves to test them I got to really dislike the sprung retainers. I did a bit of research and learnt that these can scratch the valve glass sometimes leading to premature failure. Ampeg has now reintroduced Birtcher style tube clamps to its Heritage SVT CL head and they are available as a retro-fit kit part number 2040884 which fit the BVV3000 perfectly too. So I bought them and fitted them without a problem.
I have also provided dimensions to Phil at Hotcovers who has made a lovely padded vinyl cover for £39.50 delivered.
Total cost so far = £369.53 for amp, plus £105.95 for spare set of six 6550 valves, plus £64 for valve clamps, plus £39.50 for cover = £578.98.