Thanks y'all for the advice.
I've just gone and ordered the unpotted Alnico II SMs - can't wait.
BTW I strongly recommend the SA500 for anyone looking for a much cheaper (£200-£250 compared to ££££) 335 alternative; I compared it directly with a £1300 reissue 335 in the shop and, whilst the pickups were clearly not as nice as the '57 classics in the Gibson, the overall build quality and playability was comparable, if not better, and I am sure it will sound far superior with the BKs installed.
It has an absolutely gorgeous neck, probably the nicest I've ever played and whilst, stylistically, it does look rather late 70s/early 80s japanese, which some may find cheesy, that's purely a matter of taste.
That one can get a new ES type axe of this quality, and at this price, is astounding, although it also reflects the overpriced nature of current Gibson offerings, much of whose build quality has been dubious to say the least.
Once again, ta!