thanks Fred - it does take a little practise to get the hang of the Kew system.
For those Rd. Nos. contained within very long series of BT references, I've come to the conclusion the quickest way is to make some mental comparison between the ranges of both sets of Nos. - the idea being to judge roughly where the Rd. No. in question might occur.
Hopefully, having landed somewher near the Rd. No. you want, it's then trial and error adjustment to fine tune to the exact BT No. required.
This method seems to be about the best I can think of - the altenative is to wade through every consecutive Kew page until the required one is reached - and this can be very time consuming (which is why I asked you for help last evening)
As I'm a bit thick and lacking in computer literacy, there may well be a quicker method that I've yet to find - if there is then someone might let me know, please.