It's a bit difficult here because GSV isn't all up-to-date, but if you entered Marshall Street from here, (A62), you wold have passed these signs: -
https://maps.app.goo.gl/LQ83j4vR8xq6kdE79However, at the other end of Marshall St, where you turned in there are no signs, as you say. The trouble is that this id a bit zone, and there is a sign as you drive in on the Rochdale Road here: -
https://maps.app.goo.gl/q1W2QkWsABWNhE1k9This sign is 183 metres away from the turn into Marshal St. Yes, its rather naughty, as how are you meant to read all the restricted times while driving along at 30 mph, but its part of the council's box-of-tricks to get the money in. The sign is also easily obscured by high-sided vehicles.
You should submit representations based on inadequate signage of this CPZ, there being nothing at all about this zone on their website, and a sign like this on a busy main road means it is easily missed, and can be obscured by buses and high-sided vehicles. My opinion is that the council have failed adequately to sign this CPZ, thus failing in their duty under Regulation 18 of the The Local Authorities’ Traffic Orders (Procedure) (England and Wales) Regulations 1996 : -
https://www.legislation.gov.uk/uksi/1996/2489/part/IIIHowever, don't expect them to roll over and cancel the PCN. They will send you a FOb-Off letter rejecting your reps because they know that >95% of people then just cough-up. If you want to take them all the way, you'll have to wait for the Notice to Owner, submit reps again, get rejected again, and then you can register an appeal at the Traffic Penalty Tribunal.
Are you the registered owner on the V5C Registration Certificate for the vehicle ? Under the parking legislation, the keeper has sole responsibility for appealing a PCN, although the owner can authorise a person to act for them. Particular difficulties can be experienced with hire and lease firms.