Cheers again. I think there's promise here compared to our DSL which on a good day can just about manage a 4 meg speed test, with around 48ms ping times due to fairly heavy interleaving. The other factor is that BT has our DSL classes as impacted by REIN from an electric fence, which basically gives them the right to walk away from any sort of fault. So that's quite a low bar for 4G to meet in terms of both speed and reliability.
I'll try to do some more tests this weekend, see whether I can pick up band 3 outside for example which might mean an external aerial would make sense. One thing I noticed was the reported RSRP seems amazingly sensitive to small changes in antenna position. Best orientation turned out to be angled out around 45 degrees when view from the front, but angled back by around 15 degrees. I still have two more 200meg SIMs that I can use, but it seems like I can only get one and a bit speed tests from each before they're empty.
The other thing I note is that the B525 seems pretty limited as a router, with for example a silly limit that you can't have the same SSID for both 2.4 and 5GHz wireless. I may look at either a better router, or maybe configuring the B525 purely as a 4G modem or whatever the equivalent is. If it came to it I could enable one of the two wireless channels on a separate SSID so I could connect for management purposes.
In case anyone's interested, screenshot from LTE Watch showing the effects of the final move to raise the router by a bit less than a foot.