According to National Rail Enquiries the 7.32 Southern service from Victoria gets in to Hove at 8.54. The one after departs Victoria at 8.17 and gets in Hove at 9.24. These are both direct trains - no changes. They usually go from the highest numbered platforms at Victoria at the far right side (numbers 15-19 I think). Obviously I can't guaratee these times, so please double check there's no engineering works diverting services via Timbuktu or whatever. I haven't travelled on them with a bike for a while so maybe double check their policy, but should be ok on a Sunday?
The later train might be cutting it a bit fine for a 10am roll-out. I'd rather not be riding that fast before the 20 miles have even started anyway!!
I'm happy to meet people off the earlier train at 9am - I know a mostly off-road route to the top of the Dyke.