Just phoned Pegler to ask and they tell me they've discontinued their TC5 and TC6 pumps, which were head-for-head replacements of Grundfos 15-50 and 15-60s. Which is a pity: the Grundfos ones are eye-wateringly expensive for what they are and the Terriers were more reasonably priced and Pegler are a good make. I know DABs are compatible but they do seem pretty flakey.
And, yes, I know A and B rated energy-efficient circulators are the way forward, but sometimes it's a lot easier to replace the head of an existing pump than fsck around with the whole body.
Anyone got any sources of cheap Grundfoses?