I have a 4 year old staffie, she has been spayed and is brilliant around people. In the past we have a couple of instances where she has been aggressive towards strange dogs on walks. She has never bitten but has given warning snaps! Witnessing this has turned me into a mervous wreck and now I find myself not wanting to take her on walks in case we come across other dogs! I never let her off the lead now and panic when I see another dog to the point I feel physically sick! I no my anxiety is worse than hers now and I no I'm not doing her any favours! I've tried obedience classes but that many dogs in one room is my worst nightmare! I don't know what to do for the best!

You definitely need to seek a vetinarian...
Has the dog's eating habits changed, if you can recall?

At a similar time to the aggression she developed colitis so we have changed her food but she is now settled in what she's eating!

Your dog can acquire colitis if she's anxiety-prone or has environmental causes of stress... with that being said, have you left your dog alone for a "long" period since this has all started happening..? If so, she MAY be suffering from separation anxiety...

But that can't be the case now that I think of it.
It could very well be you; a nervous wreck just not willing to take chances taking the dog out...
If you have control over your animal, take her out & let her breathe. It could very well be a phase. Dogs are just like humans, they don't want to be stuck in the house all day.
IF anything unwanted is to occur, I highly suggest that vet... at least they can tell you more than I can.

Thanks for the advice! Im convinced it is me as my husband thinks I'm nuts because she's fine with him! I wonder if the docs for me before the vet for the dog would be the best answer!

Never know..
I don't know too much about dogs, but I just wanted to help..
Hope everything works out.