Hi Mike,
I am based down in west sussex, and have had the same problem, and also paid 5 percent duty plus VAT.
there is no way round the issue new/used/antique, it makes no difference the uk customs will sting you, also watch out for the Parcel force charges.
I bought a flute from Paddy Ward, I knew that the vat and duty would be charged, so I was prepared and budgeted it in, it was sent USPS from the states, Parcelforce are the carriers for all USPS shipments, it was then held in Customs at Coventry for 4 days as they said they had a back log, it was then sent to Parcelforce, an additional charge of £ 20 pounds was added for the admin fee, this is variable, I phoned them twice to find out what was happening, and I must admit the guy was on the ball, he said they had held the parcel over as it was awaiting charges to be added, and I would recieve a letter in the next day or so, I had the parcel sent usps
express which is a joke because it took well over 14 days to arrive, anyway instead of waiting for the non existent letter to arrive I paid the charges by debit card and it was finally released to my local depot in worthing, Parcelforce have a depot in Coventry which believe it or not is on the same site as the customs office, so the parcels dont go anywhere to be checked and charges added its all on the same site, I questioned the charge and its still in dispute, they get away with it most times as when you recieve your item there charge is added to the Vat and duty and not billed separatly.
One other thing, make sure when you ring customs and excise you get an exact code/classification number for your duty charge as they will tell you one percentage on the phone and when it hits customs at coventry they apply a higher code.
Sometime, just sometimes one might slip through there beedy eyes and you will get away with it but not often, they tend to check private couriers parcel without fail, like UPS, FEDEX.
The only way round it is to under insure which is a risk, even if you have a tracked service, if it goes walkabout, you can only get the insured value back, and I bet that would require jumping through hoops.
I only meant to write a short post.
Hope that helps
sponge 
last post for a few weeks, off to Cornwall for a weeks hols and then Stanton in the USA for a weeks work, then Switzerland for a weeks work