I've gotten a similar letter in the past that I had underpaid. But I was working for a well respected pharma company at the time, and asked a colleague about it who said that it just happens sometimes, and he's gotten letters saying he's overpaid or underpaid frequently since leaving a previous job some 10 years ago.
I wouldn't be too peeved about your previous boss "messing up" your taxes. It seems like a common mistake-- even for companies who have a department specifically for Payroll/Finance.
It will all work itself out, but for your own peace of mind you could do an HMRC Self Assessment -- the way I understand it, it's basically the same as filing your US taxes, and claiming whatever refund they owe you (or the flipside-- paying the amount you've underpaid).
The way I see it, there's not really a legal issue here, as the same has happened to me where I didn't have similar issues with my employer re: late payments, having to hassle them in order to even get paid, etc. But others who have been here longer may have different experiences.
