A very late response, but thought I'd update this question in case anyone else struggles with a frequently blocked toilet. I finally got a local drainage company in and they put a camera up the waste pipe. The picture below is inside the waste pipe, looking towards a T-junction where the toilet and sink wastes combine. I do not know what all the bits are called, but the waste from the toilet flows in from the right, through a pipe with lots of ridges on it. Unfortunately this pipe was too long, and was jutting out into the T-junction. The result of this was that waste did not have a clear 100mm diameter circle to travel out through, and waste would frequently would get caught. Had a very kind plumber come out and replace all this with like for like, BUT he sawed off a few inches from that toilet waste pipe bit so that it no longer protrudes into the waste pipe.
[GALLERY=media, 108595][/GALLERY]
It's only been a few days, but so far no blockages ....