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 real commitment. And you'd need to get that commitment before the £28k offer expires.
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 real commitment. And you'd need to get that commitment before the £28k offer expires.