Goldring might be able to do the job but you would have to enquire and I very much doubt it would be the "basic" £300 fee. The difference between a Linn Asaka/Karma/Troika "standard" rebuild (complete rebuild admittedly) which is aluminium cantilever and ogura fine line diamond was £400 + VAT + shipping, for a ruby cantilever and gyger s diamond was £650 + VAT + shipping when I enquired. Assuming Goldring can get an appropriate cantilever and stylus to make it like it was it will not be £300 I suggest.
Dynavector do rebuild their top end cartidges, not sure where this starts in terms of the 17D3. They certainly give money off a new one for handing in the old so if you really liked what the 17D3 does then keep hold of it and you can use it to trade in if they will not rebuld it.
EDIT: Just checked, it is exchange on the 17D3 which is now 17DX and it drops to c.£1000 with the exchange.
Not much help I am afraid but I hope that helps in some way to understanding your options as I see them.
Dynavector do rebuild their top end cartidges, not sure where this starts in terms of the 17D3. They certainly give money off a new one for handing in the old so if you really liked what the 17D3 does then keep hold of it and you can use it to trade in if they will not rebuld it.
EDIT: Just checked, it is exchange on the 17D3 which is now 17DX and it drops to c.£1000 with the exchange.
Not much help I am afraid but I hope that helps in some way to understanding your options as I see them.