It happened that, thinking about the pond at the allotment, and noticing yesterday afternoon that the digging I'd done earlier into the clay hadn't held any of the overnight rain, I'd googled my way to Fordyce (2012) and so went onto the University's library site to see if I could use my newly-learned database searching skills to find it again... I couldn't. Which is a bit troublesome, if Google.uk (not Scholar) could get me somewhere (admittedly, entirely off-discipline) to a paper more easily than Google. Heigh ho.
On the plus side, I did find a reference to Browne (1998) on the library site, and I'll borrow a copy of that when I go back in tomorrow. In the meantime, I picked up a couple of interesting facts from Fordyce (2012). Glasgow's clay is (as I'd suspected) glacial, and therefore quite recent.
Also, there was once a brickworks at Blackhill, which might have been taking advantage of the local clay. Quick searches in Wikipedia, (no I'm not referencing them, useful as they were, come on), shows that fire clays also use quartz, pebbles of which are abundant in the topsoil. [NB -it's not as close as I thought - there seem to have been two Blackhills in Glasgow - one next to Riddrie; but the one with the brickworks appears to have been nearer Maryhill - couple of miles from the allotment.]
Fordyce, F. M., Nice, S. E., Lister, T. R., O Dochartaigh, B. E., Cooper, R., Allen, M., ... & Scheib, A. (2012). Urban soil geochemistry of Glasgow.
Hall, I. H. S., Browne, M. A. E., & Forsyth, I. H. (1998). Geology of the Glasgow district (Vol. 30). Stationery Office Books (TSO).