Anglet aire (Boulevard des Plages) which I'm pretty certain is open year round, and free (or at least cheaper) in the off peak season. That's our first port of call in that area, although no EHU.
A bit nearer Bayonne there's overnight parking at La Barre. Last time we were there the MH's were down in the dip by the river due north of the Giratoire de La Barre roundabout (see Streetview), but Google Maps is now showing an aire in the main car park so I'm not sure what's happening there. No EHU that I know of. The main car park has a sort of adjustable height barrier thingy but buses go in and out of there.
Failing that, south of Biarritz you've got Ave de la Milady aire, which has EHU but it's a bit of a sardine tin set up - can be very tight.
The first two above have easy cycle track access to central Bayonne beside the River Adour. If you go on the last weekend of July there's a huge Basque festival in Bayonne so it's busy and awash with red wine.
Thanks for the suggestions but ended up carrying on through to Tafalla or something like that in Spain. A very pleasant drive through the end of the Pyrenees in bright sunshine. On to Valencia area tomorrow and then Totana I think .