Well, something's happened!
4G signal strength now 67%/68%, so 2 or 3 bars (-91dBm/-92dBm). Nice improvement over the 50% that I had a couple of days ago, and the very small % that I had before the water tower mast went 4G.
Speeds? 26Mbps down and 37Mbps up! This is much better (in terms of upload) and suggests that my local mast has gone 4G. Though downlink might be capped to single speed at the moment?
3G from the local mast (signal strength still nice and high) is still between 15Mbps and 20Mbps downlink, and 2.2Mbps uplink.
GSM from the local mast is now EDGE instead of GPRS, so an improvement there too, though I still won't use it!
Overall, a success! Our mast is upgraded!
Relative signal strengths:
EDGE: -69 dBm
HSPA: -81 dBm
LTE: -91 dBm