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Finally got it to work after adding another layer - had thought tape that was on would be ok as it had had a tyre on before with no issue, but the second layer helped.
Also think that the valve core wasn't helping as the protrusion into the rim was pushing the tyres out - have switched to a narrower based valve with more joy.
And tried schwalbe g one speed instead of formula IRC rbcc.
Basically changed the whole lot and it worked....
How many layers of tape? And what tape?