Sorry Vulcan...I misread your post ! ( thanks goes to Hopkirk ! )
At this time of year, mail gets shifted around the country, if the local office is too busy to deal with their own stuff. Also, the postmark doesn't mean very much these days, as all mail is given an invisible coding across the stamp, by Mail Centre Staff. So all the stuff from Sussex may very well have been coded, sent up to Chester, which is a main Mail Centre, and then postmarked there.