Mark I am a little confused over which Scammell you have been offered, the 20 ton was a ballast tractor with a Meadows petrol engine wheras the 35 ton was a tractor unit with a 3.5 inch pin 5th wheel coupling for towing RE plant trailers, equipped with a RR C6 diesel, which would certainly pull your trailer, but with only 185 BHP on tap would be decidedly slow.
Why are you not going to use on of the 2 S26 tractors you have ?
John Riley