Do not use a scrubbing brush on cloth seats, you will matt the fabric

Whereas the mould is concerned give the seats a very good hoovering over first, and then use a wet & dry vacuum cleaner ( You can rent these from tool shops ) and buy an upholstery cleaner for wet vacs. Use one like Autogylm's for example, mix it to the right quantity of dilution and proceed to clean the seats. If you spray the upholstery cleaner onto the whole seat and then leave it for 5 minutes, then go back and spray more then suck it out you will remove all the mould and the smell. This time of year isnt ideal for cleaning seats as you need somewhere warm for them to dry, but if you can leave your engine running with your heaters on full blast for a bit then you should be ok, the idea is to clean them and then remove as much moisture as you can otherwise it doesnt really work, you end up with damp seats that then smell again

Hope this helps.