I've used personal trainers for years. They're great if you've got the money and your fitness is a priority. They're not really about rectifying 'laziness' or a lack of motivation; if you're concerned about bringing one in, and you intend to put them into your schedule, laziness is not your problem. That being said, getting out of bed at 7 on a freezing morning is very slightly easier if you've something structured waiting to get done - and crucially - someone to see that you do it, and that you do it properly.
Yes, by all means go running and get on a bike, or whatever. That's all great, but a trainer can push your envelope in all sorts of ways. They can assess you objectively and design exercise to fit your goals. They can push you - not necessarily always in the gym, but in a variety of environments and give you feedback on improvement. Many of them have advanced qualifications in their field - do check them btw - my most recent trainer also dealt with clients from the NHS which he was helping to bring back from injuries and other clinical conditions. They will do things with you that you wouldn't have thought to do yourself; so you don't get bored and you're challenged continually.
My back problem disappeared this year - I'm pretty sure that's because he designed a lot of exercises aimed at strengthening my core; I'll be eternally grateful for that kind of specialist assistance that saves me massive pain and days off work.
Lots of people work out at the gym. Speaking as one of them, I think a lot of us phone it in. Well, you can't do that with a trainer around. You're going to to 'bring it', as they say. There's no choice. They won't shout, it's much more subtle than that. You just find yourself working harder. And you'll stretch and warm up properly; a lot of people don't do that and then hurt themselves.
The Brockwell Lido has several who've been there for a while Pippa, and I can recommend one in particular if you'd like to DM me. Test them out, swap them around, remember you're in charge. As for value for money they weigh in at around £40 an hour. Well, that's a night on the lash in a couple of pubs and a taxi home, or a meal out or whatever. Not an inconsiderable expense I'd agree, but manageable for me and lots of my fellow office drones.
Think I might re-hire as we go into the winter, the Lido pool isn't the same with polar bears in it.