I have 225 45 r17 winter tyres on my golf that are nearly new. I have just purchased a Peugeot 207 with 205 45 r17 tyres - could I also use my winter tyres on this car?
You probably can't use them. They are too wide for the Peugeot - the max I could find was 215, and even that I think is a stretch for a car so small.
Also, depending on what rims you have on the Peugeot they may not fit on them and you'd need different rims (the ones from the Golf will not work because the Golfs usually have 5 bolts and the Peugeot has 4).
Are you planning to use the whole wheel from the Golf, or just the tyres?
The difference between the 205 and 225 tyres are that the 225 is two cm wider (that would mean one cm on each side if you change only the tyres), it will also be a little higher (about 157mm instead of 143mm).
You will have to take a look on your car and make sure that there is enough clearance between the current wheel and the inner wheel wells and the fenders. Don't forget to account for the movement of the wheel, both up and down, and as you turn the wheel. But just as @alin-purcaru says, it's likely to be too big for the car.
Otherwise, if you mean to change the whole wheel, well then first of all the number of bults must fit, then the bult pattern. You will also have to check that the ET number of the rim fits your car. Most likely this option will not work.