As Yorkie said, I'm not a driver, but I do work for them.
I imagine that they have placed this advert so that if they do decide/can run the extra Chester timetable, they can easily find staff. Saying that though, 3 months is a bit of a short turn around period for Merseyrail. Selection, Medicals, Training and Passing Out would probably go beyond the May start date. It took nearly 3 months alone for me to be asked to attend an assessment day for my role.
Regardless, it is actually looking unlikely that the improved Chester timetable will commence in May anyway due to other reasons, so they may just fill the odd couple of vacancies that exist in the meantime.
As for the driving side- you're looking at around a 5-10 year wait before you would go from guard to driver. The competition is fierce and usually favours the guards who have a lot more experience, although there are may different career paths you could take.
Typical promotion roles from Guard to:
1) Leader Guard (suppose to mentor, train and advise normal guards)
2) Resource Controller (rostering, equipment, sickness, competency etc)
3) Guards Standards Manager (recruitment, assessing, training, discipline etc)
4) Relief Train Driver (a mix of both guard and driver, originally a strikebreaker role I think- mainly guard, but with maybe 1-2 days guaranteed driving)
5) Train Driver (pretty obvious, but you're trained for both Northern and Wirral)
You'll never find train driving roles advertised externally, the majority of guards know:
1) That's were the money is
2) It's a good career move
3) They can retire in that role comfortably
So as soon as it hits the Internal Vacancy list, someone internal is bound to get it.
Full Drivers earn aprox £36k.
The shunting is actually fairly straightforward, and the only thing you have to do really is give hand signals to the driver. You're just moving stock around the depot.
So, yeah, don't expect a job any time soon, but remain positive. Merseyrail now know you're interested. Internal candidates will get priority, but that's not to say they will meet the standard required, and external people then get their chance to shine.