I have just fitted the mirrors as suggested by Planenut. They look very neat and work well on the nearside, but do not have so wide a field of view on the offside. Probably get used to that and at least they don't vibrate. So I fitted a rear view mirror on the Faibourne central pillar. Melvyn Rutter can supply this as a bolt on accessory, but didn't currently have any in stock, so they just sent me the mirror alone. Picture below is the packaging, for those who might want to source it.
Dismantling is very simple, all that was needed was a stainless steel column turned up to go through the Fairbourne screen pillar. I opened the bolt hole in the windscreen to 6mm because that was the size of dome head nut I had in stock and this size would cover the countersunk hole better.
The mirror works well in service, giving a very comfortable rearward view, but does vibrate a little - but not as bad as the original mirrors.