Hi. I've just bought a Wadkin RT lathe (smallest of the pre-Bursgreen range) - RTA 380 of 1951.
The tool post is 1 1/8" diameter and 1 7/8" long. I believe it's the same as used on Wadkin's RS lathes - and the toolpost holder pattern numbers have an "RS" prefix. However, the RT is a 6" lathe, so while it tool post dimensions are almost certainly exactly the same as an RS-6", there may be minor differences in length with the RS-8" and RS-10".
Cheers, Vann.