I have a vigor 2760 with the earlier firmware (1.2.1.3) and I have just ordered an upgrade to fibre broadband from plusnet.

I ordered the fastest package (bt call it infinity 2) with 55m download, 14m upload. What I need to know is does my 2760 handle the higher speed or does it only handle the 35m download speeds? If so what similar products will handle the higher speed?

Yes, it will be fine at 55Mb/s.

Well I now have fibre broadband through PlusNet - their "extra" package and I'm very disappointed.

1) They estimate 35-55M download speed and 14M upload but I am getting 12M & 1.2M (now slowed to 10M & 1.2M as of 5 days after install - plusnet still show estimated speed 35-55M but actual line speed 10M.
2) This speed is with their free Oneplus hub. When I try the 2760 it gives only 8M download speeds, also an ISO download from MSDN ran steadily at about 11M with the Plusnet hub, but with the 2760 it ran at about 0.5M AND the speed graph was very jagged as opposed to a nice clean "block" with the Plusnet hub.

While I am very concerned about the obtained speed compared with PlusNet's estimates, my biggest concern is the 2760 which way underperforms my expectations of a supposedly premium device!

I am on the earlier version with firmware 1.x (1.2.1.3 at present) but this should not be a problem. Looking at the dashboard, I see the processor is often at or near full load - is this the problem? And if so why was this expensive router so under-specced for a job it claims to do easily?

Their speed estimate is based on your proximity to a cabinet, but lower quality/damaged, cabling, strange routes, bad connections to your house can make the line behave worse. Processor not an issue; try different modem code, but that might be limited if you have a V1 firmware V2760.

Can you recommend a modem code to try?

Also, if I were to downgrade to the lower speed fibre would my actual speed go down proportionately? I am really not happy paying twice what I paid for adsl for a 25-50% increase in speed downloading and very little change on uploading! So I could save half the increase over adsl by downgrading to the "ordinary" fibre package but I don't know what the differences are and how they might affect actual speeds.

Not sure about code... I use a 2860 with other firmware, but ask support. I didn't understand the second bit of your message