Thanks for the replies so far. I hold a UK PPL and various other flying quals but need an FAA PPL in order to do the recreational flying I'm planning (I do not have time to wait for the 'verification of authenticity of foreign license' process). I am told that based on my currency and experience it will take a couple of hours with an instructor and a checkride to qualify for an FAA PPL (+ theory exam). In this sense it is 'initial issue' but could only be one day's flying. The remaining flying would be private and recreational, and as such is not problematic.
Here's where it gets tricky. The (AFSP authorized) flying school that has participated in my TSA/AFSP clearance (in full knowledge of my requirements) has now stated that it is not accredited to issue the I-20 form which enables me to get the M-1 visa. Unless there is some massive contradiction here, this implies that I must be able to do my flying on the visa waiver program. If not, they've led me up the garden path, wasted my $300 of AFSP fees and left me in a predicament. The school's CFI's solution of "just don't tell immigration you're here to fly" left me rather unimpressed.
I haven't been able to get a straight answer from any of the US gov depts that I have questioned so far, and each one passes me on to another dept, ad infinitum. I'll speak to the embassy on Mon, but with their phone line charging £1.20/min, I'm beginning to think it would be cheaper to fly in the UK!