Seems like they're trying to blame you for their mistake. You told them it was a new 999r so they should assume the value according to the records which they hold. They obviously didn't check.
The previous insured value was for your old bike therefore despite you saying everything else is the same, it's rubbish to say they'd assume the same value also. They have the info for vehicle you declared and they just didn't use it.
If you have some evidence of the quote you got on the phone then you can tell them to stitch it! Otherwise it's a case of trying to "trick" them into admitting the number they originally quoted - They cannot cancel your policy on the grounds of their mistake, they'll threaten to but you can tell them to foxtrot oscar.
Just remembered.... same thing happened to me many years ago on a car policy. I ended up telling them to sod off, and threatened to take action against them if they tried to void my policy. Rather than chasing evidence of what they actually quoted, I asked them to prove that they quoted the extra - they couldn't, therefore no amendment could be made either way, and according to the smallprint the vehicles value is assumed as market value unless stated otherwise.
Ball's in your court here mate, don't let them scare you. You can either take a moral stand and offer to come to some sort of agreement, or you can demand they prove the agreement made on the phone for an extra £6xx.
[Edited on 4-5-2004 by NBs996]