I had a guitar (a Gibson) from Canada, and the seller helpfully, but cheekily, filled the customs form out saying it was worth $100 (as opposed to £700!!)
I was charged by invoice from the shipping courier about a month later - as I recall it was £12.
If you go on the HMRC website or google it, there is a table of import charges that explains exactly what % you will pay - if (and it is a big if) it gets caught by customs.
If it is held by the post office for payment (ie if it was not send by a courier, but by US mail) you may also need to pay them a handling charge - I recently had some camera gear through from the US - didnt pay any import duty, but had to pay the post office £7 just for holding on to it - that was annoying!