I managed to get my mothers ISP to send out a new DSL router, it is a Technicolour 582. My brother in the UK went round to help set it up.
It is supposed to support Wireless-N, but my brother said my mothers iPad2 only offered them one SSID to select from and he couldn't find out in the router setup page where to enable Wireless-N. Perhaps her ISP pre-configired it for Wireless-G only, or maybe Wireless-N only?
Interestingly the ISP had preconfigured the wifi to manual, channel 1. I think this may be because they knew we had issues with using an iPad and they had helped other customers before by manually setting the channel. I have no idea what channel her neighbours are using but as mostly seniors live around here, hopefully the airwaves are not very congested.
Anyway, the good news is that (at the moment at least!) her ping and upload times are much improved. My brother ran SpeedtestX HD, connecting to a server in London and it reported these numbers:
Ping: 25
Download: 5.02 mbps
Upload: 0.25 mbps
Ping is much improved (got numbers like 473ms before)
Upload although poor is still almost twice what it was before
We had a short Facetime session and did not get the black screen/poor connection message, so fingers crossed that the connection is good enough for now.
Clearly, Apple still have some room to improve the wifi peformance, as evidenced by the fact my brother-in-law's got 0.350mbps on his laptop before), but hopefully this will enable my mother and I to Facetime again.
While my brother was helping, I got him to run traceroute to google.co.uk and also to the Facetime/ichat server. I can post those results if anyone is interested. I dont know if it tells me much, except that one of the nodes within my mothers ISP looked a little slow at the time.