The forend on16 ga WJ Jeffery circa 1930’s has slight play when attached to barrels. The barrels are tight to action without forend attached. Is this a hard fix and any recommendations as to whom could repair?

Depending on the direction of wobble, it is most likely loose on the back of the barrel loop. This was traditionally tightened by peening up the forend iron on the internal surface but is now more usually done by building up the bearing surface on the forend iron with TIG or laser weld and then filing to fit.
It is a very simple job for a trained experienced double gunsmith, should take less than an hour.
Beware, welding the forend iron will lose you any CH colour in that area.
Also check that your forend barrel loop hasn't come loose in the barrels, it happens!

Ducks,

Youtube video of the late Jack Rowe applying the peening method referred to by Toby Barclay.

https://youtu.be/nuGBOtnDZlY

Jack Rowe tightened a loose forend for me by peening the iron. I watched him do it and was amazed how simple the process was. Worked great. Took him a few minutes while he would stretch it a little and test it and then stretch a little more. He had to straighten the iron a little since it bent a little with the peening.