you can just enter, but to actually get a start you need some decent performances in open hill climbs.
The field is limited to 150 riders for the national champs. Usually there are more riders than spaces available. At Long Hill last year somewhere in the region of 50 or so (IIRC) had their entries returned.
So - to make to the top 150 you need to have ridden some hill climbs and got reasonable results in them. However, if for some reason the race does not attract 150 entries then everyone gets a ride.
It is on Rambottom Rake this year which is a classic nationals course, so it will be full and then some.
The organiser will select the best field according to performance in the event of >150 entries.
Come do the Carlton Bank hill climb. it's really enjoyable and fun