Hi both, thanks for your replies. I think you're probably both right. But you know how it is with these things - you tend to prepare yourself for the worst!
Another reason I was slightly hesitant to do this is because I have an Irish passport (though I'm a UK citizen). It's not an issue in terms of my eligibility to endorse the application (you need to provide a UK or Irish passport number). Rather, I was hesitant because the UK government doesn't have details of my current passport as it's relatively new - having been convicted of an offence that landed me on the SOR, they did have details of my previous one. Since my time on the SOR ended a couple of years back, I haven't had to give them my new passport number, nor do they have any legal right to ask for the details. When I got my new passport, it felt like a fresh start. But the paranoid part of me can't shake the feeling that this is a pretty easy way for them to get this info, and that somewhere, somehow, someone will add me to a "watchlist" (or at the very least, a "person of potential interest" list). I have zero confidence in the UK governments' data protection, and would much rather avoid this kind of contact with them altogether! I know this probably sounds overly paranoid but the truth is that we do live in surveillance state in which the collection of an individual's data is a routine activity.