I'm just wondering if anyone can tell me how reliable the estimates for fibre speed from BT are?
R0nski's given you the right answers. Here's my personal experience:
My current line is 400 metres, with an estimate of 55/18. It initially achieved 80/20, with maximum attainable speeds of 88/26 (reported by the modem). Over 18 months, the crosstalk has increased, and the modem now reports attainable speeds of 72/25.
I'm still actually sync'ed at 80/20 (the last resync was 6 months ago, when I could still get an 80Mbps downstream) but with a noise margin of only 3dB. I guess it'll drop one day.
would crosstalk affect properties 10 metres from each other so differently?
Crosstalk is a funny beast, and depends more on how close the copper pairs are within the cable - which ultimately depends on the manufacturing process - and which households actually take the service.
It means that there *could* be a dramatic difference on any two properties no matter the distance between them.
However, the effect is random and unpredictable - so BT's checker cannot take crosstalk into account to that extent for every individual line. Instead they know what is statistically the worst case effect for each distance, and apply that to every prediction/estimation.
So, if both your lines are the same length and made of the same material, the prediction ought to be the same too.
Also, is it possible that my line (residential), and my neighbours (business), could account for the difference?
Business/Residential wouldn't account for the difference, but some other aspect of the line certainly could. Perhaps one line uses an aluminium cable in the return to the cabinet. Or perhaps it returns a different route - both would affect the prediction.
As both lines go to the same pole, you'd think this was less likely, but it isn't impossible. The possibility of a database error is there, as mentioned by R0nski, but isn't a certainty either. You might just have to pay your money to find out.
currently I'm considering updating to fttc but there's a chance I'll be moving house before the end of the year, I would risk getting into an 18-month contract for 20mbps upload speeds but 6mbps upload would probably not be so worthwhile
We went with Plusnet's fibre-extra package on our old house, and later moved within contract. The fact we moved wasn't a problem - we just connected to fibre at the new property and stayed on the same contract. If we'd moved to a non-fibre area, we'd have been forced to drop to an ADSL package with them, but wouldn't have been penalised - though I have no idea if they work the same way now.
Plusnet also have a nice touch if you need to break out of the contract early - they don't charge you the full monthly price for the remaining period. Instead it is around £5-6 per month remaining (you can find the actual amount on their legal T+C page, I think.
If you enter just the postcode (in the one I linked to) you can select one address, see the results, click the browser back button, select the next and so on.
Yup - this process works well, having acquired the postcodes from Royal Mail's postcode finder (using just street name and town).