Don't have a 30, got a 38 but we replaced the original calorifier with a Quick one a couple of years back. I measured the original one and the space if was installed in before buying the Quick one (IIRC its a 20 litre one); if was slightly larger than the original but would fit in the space vacated by the old one. What I'd neglected to do was to measure the gap into the space for the thing; marginally too small (like say about 15mm). I used a Dremel to cut away a thin strip of GRP to widen the gap and the thing went in no trouble after that.
There are so many calorifiers out there, its probably better to do a search for one of more or less the right size and be prepared to widen the gap slightly if required if you find one that'll fit the whole but not the entrance way.