Court action for rent arrears.

Rent Arrears

Family of 4 adults, parents and two adult children. Arrears over £7,000. RLa advisor did not know claim forms are for only two people -so have to claim twice for this group. RLA advised keep claim under £5,000 in small claims court, then if successful claim again for remainder. Filled in for parents claim at £5,000, but what do I put on the second claim for adult children. £5,000 as it will be up to £10,000 by time evicted or remainder of what is now owing remaining £2,000.
Also what interest can I charge - is it 8%?