After several weeks with numerous calls and many thanks for the patience of both BT and Vodafone Technical support discovered there is a light at the end of the very dark SureSignal + BT Business Infinity tunnel.
I have successfully been using Suresignal for many years with BT Business Broadband with various BT hubs. Before upgrade to Infinity was running SS 2 with Business Hub 3 ... when engineer swapped in the Fibre modem the Business Hub 3 was not changed and for the first about 6 hours the SureSignal did work.
Then it stopped and no amount of port opens/DMZ's/MTU changes would cause even the slightest murmer.
The next few weeks involved swapping out every piece of equipment ... no joy with any Business Hub 3 combination, port opens, DMZ's, open port static IP's, MTU - no joy with any combo - it didn't make sense. I did notice that during SS initialization the typical mass download from the internet was not occuring (as detected by internet download activity on the infinity modem) so something was blocking/filtering the packets.
Then I managed to lay my hands on an old BT Business Hub 2701HGV-C, the 2-Wire Silver/Black box with the additional WAN ethernet port covered with a sticker.
Plugged it in .. default settings ... and within 30min the SS was working fine, NO port adjustments, NO MTU changes, no Static IP required so the UPnP does work.
Then tried a few of the other options mentioned on the forums to see if they did have a positive / negative effect.
Summary of options that appear popular in these forums for supposed reasons for failure:
The Business Hub 2 was set to PPPoE mode - no choice - (has to be and any downstream device e.g. SS should be unaware of the whole PPPoA/PPPoE anyway)
The Business Hub 2 had an MTU of 1492 ( see below for comments)
All port opens was done via UPnP and there are various other UPnP devices on the network all trying to grab their own ports.
I then played around with the MTU down to 1472 first just to see if it broke the SS ... NO... then up to 1500... SS still working. So MTU doing what it was supposed to i.e. if larger packets do get sent > 1492 they are merely fragmented and sent as multiple packets and as long as every box plays nice the fragments will reassemble at the Vodafone end without error. Like the whole PPPoA/PPPoE debate downstream devices are unaware of the MTU ... BUT .. it can severly affect performance.
Now the 1500 figure seems to be the true maximum that Infinity can handle, you need to subtract the PPPoE header size ( 8 ) from that to find the highest usable figure = 1492.
To work all this out ping -f -l 1472 remote ip address should ping OK but ping -f -l 1473 remote ip address should end up fragmented. There is further overhead for the 'ping' of '20' 1472 +20 (ping) +8 (pppoe) = True MTU = 1500. (set router to 1492)
Further testing of these to prove the splitting was easily done by running a speed test. With MTU set to 1492 very good throughput achieved, with MTU set to 1500 the speed test performance was halved... proving that the packets were fragmenting very badly.
So the key behind all of the this may possibly be that the BT Business Hub 3 appears to be 'cleaning' out any fragmented packets thinking they are some sort of security violation instead of the innocent correct packet fragments they really are.
So what is the downside you may ask .... lack of wireless 'n' on the router fortunately almost everything wired anyway as wireless too congested around here anyway and 'g' enough for what's needed.
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