I finally undertook a (partial) relearning of Templot to produce a template for a loose-heeled straight 1 in 4 turnout.
This gives a radius of 19.6″ compared to 17.3″ for the A4.
Much more productive for me was to produce a template for a loose-heeled 1 in 5.
Compared to an A5, this saves approx 23mm on the length to the crossing nose.
It even saves c5mm on the length for an A4.
This enables me to replace the planned A4 with a loose-heeled 1 in 5 (Thanks Martin W!).
OK I have to curve the siding slightly to compensate in the restricted space available.
But this is well worth it; the turnout radius will be 30.8″ and should comfortably accommodate tender locos at low speed.
Before this I was contemplating a restriction to short wheel-base tank locos.
All I have to do now is build it!
Thanks for all the responses.