My understanding is the following:
24th April - Moto Arena dirt or silverstone astro
15th May - Eden Park astro
26th June - NDOR astro
24th July - Adur dirt and astro
21st August - Coventry dirt
11th September - Moto Arena dirt or indoor multi surface
25th September - A1 Raceway dirt and astro ??
hopefully the respective clubs will correct as necessary