The first thing that came to mind when I read your post was: imposter syndrome. Have you considered whether that might be what you're experiencing?
Of course, there ARE people who are currently in positions where they are overpaid compared to what they could get on the market. There ARE people who interview badly. There ARE people who have no network to draw on. But there are also MANY, MANY people who think these things erroneously. You may not be a very good judge of your skills (imposter syndrome can be a HUGE part of this) & your value on market (this is especially true of people who haven't interviewed in a while and therefore don't know their market well). Just something to consider. I thought I probably couldn't increase my pay on the market before I left my last job, but I increased it by £5k. I didn't know my market or my worth at all until I started interviewing. I also thought I was unprepared for interviewing but it turns out I interview great!
Here's also a thought on network: I don't actively network. I don't attend events in my field, I don't really contribute to it online, I don't have many social connections who also work in my field. But when I started casting my mind around, I realised that through my tenure at my then-employer I actually had several good connections in my field -- all the people I had been coworkers with at my employer, who had themselves since left the company. If you've been with your company for a while, this might be a good thing to consider. Do you know people who have left? Especially if they have seen your work in action? I wouldn't worry about having kept in contact with them in the meantime, with the aid of linkedin it's very easy to reconnect -- the important thing is just that they thought well of you while they were there. I didn't end up using any contacts in my job search (except as references), so I can't comment more on the value of networking.
A lot of tips about the other stuff is very field-specific and location-specific. I would say: recruiters can be good, but be picky. A lot of them are just trying to flog you the same jobs and will be pushy. Don't upload your CV to job searching sites or they will swarm you. There's no particular need to START with recruiters unless it's how things work in your location/field, you can start by searching job listings and making specific applications.
In job interviews I always ask culture questions & questions about the work: how big is the team I'd be working on?, what's the makeup of the team (I usually work in multi-discipline teams so here I mean by e.g. job title)?, how are projects managed and scoped?, what's the most interesting project you have ongoing? Some of these are field specific, some can be adapted.
Despite being American -- and therefore some advice needing to be filtered through a make-corrections-for-UK-culture net -- my favourite job hunting/work/being professional advice has always come from
Ask a Manager, who writes a great column.