Sorry to have been ignoring you guys lately - needs must v. busy.
On this one, different providers have different ways of arriving at their prices and you can go mad trying to match them up. The worst is the opening. The "official" line on FTSE is that yesterdays close=todays open so there is never an opening gap. Some providers follow that official line whilst others try to bring more reality to the table by using 8:00am first trade as the open, others by 8:00am quote.
Most SBs just open at the level their algos have got them to at the time, then do immediate catch up to actuality (untradeable usually).
All a minefield really.