Other than the ID that BatBoy has found for the current fibre twin, FTTC cabinets do not have a number, as in 29. The 29 or similar everywhere else is always the PCP number. Very occasionally where a new PCP or combo unit is installed for EO lines IIRC there can be a prefix letter with the number.

If you are on PCP 29 and its fibre twin is full, (which has been established), and a second fibre twin is planned for PCP 29 as Ribble says (and knows), then you just have to wait for that second twin, or a port on the current one being freed up.

Given that few ISPs use the "Waiting" system, though some smaller more expensive ones I think do, you either hunt for one or check first thing every day for yours showing as available and order immediately. EVen then you could be too late as someone might have beaten you to it but it isn't showing up yet.

Thanks everyone for all the replies.

I've set up a page watch on the BT ADSL checker, as well as coded up a quick app that checks the OpenReach "Superfast" site every few minutes and sends me an SMS when the result changes to accepting orders.

Just a waiting game now.