Totally understand where you're coming from Danny - new equipment is worth saving for, and it's to your credit (so to speak) that you haven't succumbed to the easy method of putting the amp purchase on a credit card. The amp will sound better when you know it's yours and not on tick.
Anyway. I presume the reason you want an account in Hertfordshire is because you want a high street branch rather than internet banking? Or maybe it's because you want to support a local business? If the latter, the Harpenden Building Society might fit the bill, but their interest rates are nothing special.
For a decent independent comparison of savings rates from most if not all banks and building societies, you could check out the Money Made Clear website at http://www.moneymadeclear.org.uk/tables/ and click on Savings.
Effectively none - other than tax! A cash ISA is simply a savings account which is not taxed. The interest is paid to you in full, so they are slightly better for you if you are a taxpayer. If you are a non-taxpayer then there is no difference - you can arrange with the bank to have your interest paid free of tax anyway. The other main difference for you is that there is a maximum contribution you can make into an ISA each tax year, which runs from April 6 to April 5 the next year: £5100 this year, so you are well within that figure.
You will notice that I have talked about cash ISAs. The other kind is a stocks and shares ISA, which can hold investment funds (stocks and shares) - but investments such as those are not suitable for your short savings timescale. Just to complete the picture, though you can invest a maximum of £10,200 into a stocks and shares ISA, as long as your total investment into all kinds of ISAs this year (ie including what you have in a cash ISA) is no more than £10,200.