On the positive side, I can't complain that they're not swift ...
*From:* "Terry Deere" Terry.Deere@dft.gsi.gov.uk
The website does cover your case - there is an exemption allowing achild over 3 to use a rear seat adult belt where having two other childseats means there is not room for a third.
ABfor Terry Deere
Dept for Transport
Road User Safety Branch 2
Great Minster House
76 Marsham Street
... and my response ...
Dear AB for Terry Deere,
This is getting frustrating.
As I have said, the problem is not that there is not enough room for a third booster seat, it is that if we put two booster seats in there is then not enough room for the third child, with or without a booster seat.
Please can you advise me what to do in these circumstances.