Marshall provides back-up to Brad Friedel and is in his second spell at the club.
The 35-year-old whose current contract ends in June, confirmed: "I'm just about to sign a new contract so it is very pleasing because I really enjoy being at this football club."
He added: "It can be frustrating (being back-up goalkeeper) but I've gone from number three to number two.
"It will be hard to dislodge Brad Friedel who has been phenomenal and it will take an injury, not loss of form.
"It's not being defeatist, it's being realistic."
Source: PA
Source: PA