I just finished refurbing this Claud Butler

[mikeymustard]

Oh yeah, tyre sizes can be very approximate indeed: I bought a claud -strangely - with a pair of bontrager 1&1/4 that looked like flippin' balloons!

 

[Oldfentiger]

Update - problem fixed

I sourced some Raleigh branded Cheng Shin tyres from SJS Cycles.
They're 27 x 1-1/4 and they are 29mm maximum width, so now I have ample clearance between the chainstays.
That's a full 5mm narrower than the Michelin World Tour tyres.
They are dirt sheap @ £5.99 each so I hope they'll be OK.
I used to use Cheng Shin tyres on a Pro Kart for endurance racing and they were fine.

https://www.sjscycles.co.uk/tyres/raleigh-sports-tyre-27-x-1-14-inch-32630/