Because I tried to answer on a closed thread, lol.
"there are no changes in your syn allowance as far as I'm aware when you do and sp day eg you can't have crisps (carbs) or wine (grapes!)"
No there's no change to your syn allowance at all, but there are some foods that are lower in Syns than if everything in them was synned because they include a free food allowance as well, yoghurts are one of them, or things like those McCain frozen jacket potatoes that are 1 syn.
It's the free food in them that causes the issue because to do a proper SP day you shouldn't be having the free food, you can't syn the free food and so with those Syns you're having free food on top effectively.
It's not going to be a huge amount, but if you're having SP days to boost results (rather than just because that's how you prefer to eat) you're doing yourself a bit of a disservice to have those free foods in synned foods as well and to then have the extra b choice, you'd be as well just having a yoghurt with no Syns if you want yoghurt and having one b choice.
Nobody is going to come after you with a big stick and you won't suddenly gain weight...but for the best results you want to avoid synned products with free food in them on SP days.