Just want to tell you all about this in case it's of use to anyone.
For the last three years we've been to the annual breaking the barrier/surfable event at Bigbury (but it's run at other sites as well) It's a big event organised by Lifeworks who do a lot of bits and pieces with children/young adults with learning disabilities (respite etc).
These one day events are free - this year we had to pay a deposit of £20 but we were given that back.
They've also teamed up with surf schools to provide subsidised lessons throughout the year and now a few free events as well. Ds1 went to one last week at Discovery Surf school. It was run in a similar way to the big events although with one person per child (rather than 2) and normal boards rather than the big long double ones they use for the big events. There were 5 children in the group ranging in age from infants to teens.
Ds1 absolutely loves it. The instructors are really good. I don't know that they have any particular training in LD's (Lifeworks who organise the big events obviously do) but they're a very accepting bunch and don't seem to have any problems. They've all been great with ds1.
There's a www.surfable.co.uk/page005.html list of surfable surf schools on this website]]. Might be useful if there's one near you or if you go on holiday anywhere near one.