....finally gotten round to getting some decent motorbike boots
Please don't tell me what you were wearing before... :yikes:
... the stiffness means that it's hard to change gear - maybe I just need to get used to them?
If they're leather they'll soften up a lot with use. Make sure you keep putting leather treatment on them, at least every week while you're using them but hopefully more often in bad weather and you'll find they give much better service.
... I can't rest my foot on the peg without putting my rear brake on.
Adjust the lever? I have to adjust the levers on my Hayabusa if I wear my Alt-Bergs on it. On that bike I prefer to wear something lighter but if the weather's bad I have little choice.
I like to keep my foot over the rear brake because the windy country roads around here are so unpredictable, there are a lot of times I have to break going round a corner because some idiot in a landrover comes speeding round the other way so it's quite a big problem. Do any of you have any suggestions?
Apart from the obvious "slow down a bit", you mean? You probably have enough experience of those roads now to be a better judge than anyone reading here of what hazards they might hold. I'm not sure I'd like to rely on the few extra milliseconds I'd get from covering the brake lever to keep me out of trouble, I'd be far happier knowing that whatever comes around the next corner, and however fast it's going, I'll be able to avoid it. Maybe it's time to start thinking about an advanced riding course?
Also, do you think my feet being slightly loose in them would affect the safety? They are my normal size but they do feel a tad big...
You need to make a judgement about that. A loose boot (or any other bit of kit) is not as safe as a well-fitting one, it can fly off in a crash, and then you have no protection from it when you most need it. But most people who ride all year round won't be able to justify having several pairs of tailor-made boots and will usually get a pair big enough to take winter layers which will not be worn in summer. That will make the boots loose in summer, often when most riding is done. Ideally, whether it's a boot or glove or whatever it is, it will have a good range of adjustment in the fastenings. Things that are too loose flap around, which can be distracting and surprisingly tiring. They can let bits of you get cold and wet. Things that are too tight restrict circulation and let bits of you get cold. As you've found, a boot (or any clothing, gloves, helmet, whatever) that doesn't fit well or is uncomfortable can make it more difficult to control the machine.
If your boots fit better with an extra pair of socks, or even two, then it will be safer and warmer. But if the boots are way too big then you really ought to change them. Aldi are pretty good about returns, but I don't know what they'd say if you tried to return boots that you'd worn just because they didn't fit well.