Yes, you will have Duty to pay. The exact amount depends how the seller completes the Customs Invoice Paperwork, and if e.g. he doesn’t include and HS/Tariff Code what the Customs people decide to classify his goods as based on his description. I would expect it to be considered sporting goods – so I think that is 3.7% duty. You will pay duty on the total cost (inc shipping)
As well as Duty, you will need to pay VAT, and in addition the courier will usually want a fee (typically about £10) for their efforts in collecting the above!
So if the total cost is £350 that will mean you need to be expecting a bill of about £100…