Sanity over 3 child seats law

It was beginning to look like I needed to buy a wide gas guzzler in order to get my (soon to be 3) kids in the car. I've got a 4 year old and a 2 year old and one on the way. Can you get 3 child seats across the back? No chance. Nor is 2 child seats + one adult in the back any fun. I discovered that sanity prevails; in that if you can't fit 3 you are allowed to just use the adult restraint (see exceptions http://www.dft.gov.uk/think/focusareas/children/childincar ). Obviously safety is important, so I'm now looking in to belt adjusters and a cushion or similar that will fit for the oldest. When there's only one adult the eldest will need to use his seat in the front - so there's a fair bit of faffing about to be done.