As above really do the 2 lower harness mounts have to be in line with each other or can one be slightly forward of the other and take up the difference by adjusting the harness.

No I don't think you have to,

However reasons for failure under SVA;

* lap belt does not rest on pelvic area

* lap belt section is not rear of a vertical line taken at the point where the belt passes over the body


* a seat belt anchorage which is of inadequate strength and likely to fail

How about adjustment?
Mine is difficult to adjust when in the car as there's not a lot of room between the buckle and where the belt comes through the seat, but you can adjust it if you're not sat in the car.

I should be fine with those SVA points one of the mounts is only 5mm further forward then the other. They wont be quite as far back as the back of the seat but will probably be about 3 inches forward of the very back of the seat. Will this be ok?

* lap belt section is not rear of a vertical line taken at the point where the belt passes over the body

Where is this taken from? The front of my body could well be further fowrard than the front of other peoples body!