I think the holiday makers would have supplied home addresses to the travel companies, so on the whole they would have been easy to contact.
Not everyone present in PDL that day / night would have been on a holiday package, nor even on holiday. That said, even if they managed to send out some questionnaires, I find it to be a good step to take.
My question is whether the T7 would have also received one, when they'd already been interviewed by the PJ.
An alternative is that the UK police described the T7 statements as responses to questionnaires, as the interviews may not have been described as such in a UK context. If that's the case, then my recollection of a couple of statements, half in correct English and half in "raw" form, is possibly all they had to go one.
I could be quite wrong, no idea. Howevder, another reason for thinking that the latter might be the case is that if you look at the translator fee documents in the files, I don't see how they could have covered the supposed volume.