I've got a cold.chesty cough, runny/blocked nose etc no fever though.is it ok to do a high impact exercise class?

If breathing OK: If you are breathing ok and not feeling short of breath with the exercise, it's fine. Try and do it in a well humidified area so your airways don't dry out, and drink plenty of fluids, preferably water, to keep yourself hydrated. If your body is too tired to do what you are asking of it, it will tell you - listen to it if that happens.