Lots of people advise job hunters to be skeptical of counter-offers. For example: http://corcodilos.com/blog/7874/should-i-take-a-big-counter-... The job offers you got suggest that your current employer has been happily underpaying you by 50% of your market value. Also, you're comparing a "hard offer of £28k" with " informally agreed to better the £28k offer". Don't turn down the hard offer until you've gotten a rea…
It's true that it seems like I'm being underpaid - I wouldn't say they're happily doing so. We're a young start-up struggling for cash and to make ends meet. Hopefully the funding round will alleviate that to a degree. Also thanks for the link. I hadn't thought of it like that. From my perspective, £18k (and indeed £28k) is a lot of money. I'm 19 and I'm getting paid to code, I know it's naive but I think it's awesom…
But 18k doesn't sound far off the salary I'd expect an inexperienced junior dev to make. Things that might get you a higher salary for this kind of role are a degree or prior experience. If you have prior working experience (even freelance) then you probably are getting a little too low.