Posted: 27/07/2013 at 17:27
I'd need a lot of convincing that any of those things can have any impact on ripening. I started mine late this year (sown on Good Friday) and they are ripening just fine. I have 10 varieties including 'heritage' Italian, so the seeds are likely to have come from all over the world.
What is different this year is the compost which has been available. Most of the common suppliers are using new formulas because of the upcoming restriction on using peat as a base. Luckily, the one I use hasn't changed yet. Are you using commercial compost and if so does it appear any different this year to previous years? Are you growing more than one variety so we can eliminate a bad batch of seeds? I feel sure this can only be a cultural matter.
PS, once the temperature goes above about 27C, this will slow down growth just as low temperatures will, so the recent hot weather may not actually be helping!