or maybe at least explain why our fibre performance is the same as our non-fibre performance
Comparing fibre performance with non-fibre performance is like comparing apples with pears.
ADSL depends on the distance from the exchange, with a lower top performance and a greater overall reach.
VDSL2 in FTTC has a much higher top performance for those close, but degrades *much* faster than ADSL. Some documents glibly state that it degrades to ADSL-like performance after 2km. Unfortunately, practical experience seems to show that FTTC runs out anywhere between 1km and 2km, depending on the kind of metal in the line (aluminium is worse than copper), the diameter of the metal, the age, the number of joints, and the exposure to the elements.
Meanwhile, ADSL can reach over 5km, but from the exchange rather than the cabinet.
For someone who lives far from their cabinet, but where the cabinet is pretty close to the exchange, they could well find that FTTC is *slower* than ADSL.
The cabinet that serves our property is 1.5 km away by road but the line back to it is around 3.5km due to the circuitous route that it takes.
Are you sure? We've certainly had people report your kind of speeds for distances between 1.5km and 2km, but we've never had a report of anything that works *at all* at 3.5km.
and is so much worse than that of our neighbours.
We have come across cases where an individual's line *has* taken a circuitous route, while the neighbours has not - this usually happens when the copper is degrading, pair swaps have happened, capacity is reached etc. I'm not sure it is common, but it is possible.
I’m looking for advice about how to proceed with our fibre broadband. We get poor speed (<2 Mbps) but our neighbours get relatively acceptable performance (12 Mbps+)
At this point, the usual advice would be to start arming yourself with more knowledge about the state of your line, and the statistics that come from the modem. This is hard enough when you have to learn about what the statistics mean, but made much harder when BT have locked access to the modem so you can't get to the statistics.
Take a look at this post
I made on a recent thread. It has the same advice I'd start with here.
We upgraded to fibre broadband on 6 December 2012 and the speed we got was 5.0 Mbps. This reduced by 200-300kbps every day until it has settled at 1.8 Mbps.
That suggests that DLM has been intervening to gradually increase the interleaving & error protection, which has gradually robbed you of bandwidth to feed the protection.
That, in turn, suggests that you have a long line *and* are suffering from interference - the most common type being crosstalk from other FTTC customers. Have your neighbours seen their speed decrease since you connected? You could be interfering with their service too...
I feel that we are being fobbed off , that BT Openreach are simply sick of us and that the engineer is just covering for his own lack of knowledge and/or incompetence.
It is definitely hard work to get a fault FTTC line sorted out - it seems to be harder than plain ADSL, certainly.
However, the things that get in the way tend to be the Openreach systems & rules behind the scenes rather than the engineer who physically turns up, but only has a limited amount of time he is allowed to spend.
Unfortunately, you are right at the point where BT can indeed tell you that fibre isn't suitable. To get them to react differently requires you to do most of the running now...
I also don’t understand how neighbours both sides of our property (one closer to the cabinet, the other further away) who must have 99% the same topology get perfectly acceptable broadband performance while we do not.
The biggest issues occur with aging copper infrastructure, where some of the lines are not really suitable for fibre, and that is running at capacity. The usual solution to a dodgy line is to swap to a spare. But if there are no spares, then you get to keep whatever is left... and Openreach aren't especially keen on replacing the multi-pair cabling from cabinet out to the distribution point.