I have found a work around on another web site, and it has done the trick. Using a sharp pair of snips, I have split each of the bulb holders and this has given enough freedom for the new bulbs to screw in. They are nice and tight so I think all will be well. The idea came from the classic LED bulb supplier. Apparently his LED bulbs have the same problem because (he says) of standardisation of bulb bases.
https://www.classiccarleds.co.uk/produc ... 0672&_ss=r I might have bought a set of LED bulbs from him, but he only does them for the later cluster with twin indicator tell-tales, and mine is the earlier version.
Thanks for the responses. I can reassemble the unit now and get the dash back in the car.