400 Bad Request

I am repeatedly encountering a message with this title and reading :-

"Your browser sent a request that this server could not understand.

Size of a request header field exceeds server limit."

Particularly when trying to use the National Rail enquiry journey planner. It's preventing me booking tickets.

I have tried a couple of ways to clear the problem without success. Can someone provide me with the up to date definitive way to get rid of this debilitating problem please?

Grateful for any help that can be offered.