Further to the note about masts and the availability, or lack thereof, I now how a 'mast' procured from a local Home Depot.
I was advised that folks use fencing poles as masts, and indeed Home Depot stocks galvanised metal poles in the fencing section. When I mention the purpose however, the Home Depot associate who pointed out the fencing section to me also noted that the electrical section had similar poles used as conduit. Taking a look at these, I noticed that the conduit came in appropriate sizes (2", 1.5" and 1.25") and was more or less the right length for my requirement. So, I picked up a length of 1.5".
Unfortunately, on getting the 1.5" back, I found that it doesn't quite fit in the sleeve of the tower top section. It does fit within the diameter of the sleeve, but annoyingly the sleeve is constructed with a bolt halfway down its length in order to secure the mast. There's a small plate protruding into the sleeve that contains the thread for this bolt which partially obstructs/restricts the sleeve to somewhat less that 1.5". Thus, although the tower documentation (er... flyer) says it accepts 1.5" diameter masts, this would not be possible unless this bolt/plate is removed.
So, I returned to Home Depot and picked up a 1.25" pole of the same length. This does appear to fit in the tower and will be secured with the aforementioned bolt. To my eye, it looks a bit weedy - especially compared to the 2" aluminium mast I brought from the UK - and I'd far rather this was of aluminium construction, rather than galvanised steel. Still, this appears to be my best bet, given all the advice - I guess it is what it is and I'll just get on and use it.