The registrations of the Rhyl Enviro400s are as follows:

4536 CX14 BXV
4537 CX14 BXW
4538 CX14 BXY
4539 CX14 BXZ
4540 CX14 BYA
4541 CX14 BXD
4542 CX14 BXE
4543 CX14 BXF
4544 CX14 BXG
4545 CX14 BXH
4546 CX14 BXJ
4547 CX14 BXK
4548 CX14 BXL
4549 CX14 BXM
4550 CX14 BXN
4551 CX14 AZG
4552 CX14 AZJ
4553 CX14 AZL
4554 CX14 AZN
4555 CX14 AZO

Thanks to Smithy from the Merseyside Dennis Dart Website for this information.