Thanks for all the interest just a few points to add.
Yes you can hire bikes you need to let me know what size you want so I can book them.
The coaches will not have time to teach pure novices how to ride a track bike. I will put on a session at Welwyn before the 13th April which you will need to attend if you have not ridden the track before.
I do not know about parking at the velodrome at the moment but will do my best to find out.