VAT group accounting

I act for a group of 3 companies at the moment they each have their own VAT return, our firm does the monthly accounts and VAT even though the FD is an accountant.

Two have the same quarter ends and the other doesn't.

We are looking at a group registration. I have got the forms etc but can anyone provide any practical suggestions:

1. What are the main advantages - from what I can see it is
- preparing one VAT return

- inter company transactions don't have VAT on them - correct?

2. Downside

- Messy accounting as I have to add up VAT across the 3 companies and transfer VAT around

- joint and several liability if one goes bust

Any other suggestions?

Director is a qualified accountant and has done this in his past job so is very keen to drive it through and I am not

Jessica