VAT treatment of repair on goods - Evidence needed to prove establishment

Hi, Our contract is with a UK client for the provision of goods (supply of equipment) and services (maintenance and repair of this equipment) across their sites - where some are within the UK borders and others more than 12 miles away in North Sea. There is no mention of these individual sites in the contract, but the goods and services are called off via purchase orders which specifies which site the work is done for. In some instances we receive goods back from off-shore which are repaired on our UK site and once repaired we return the goods to a UK address. Our clients argument is the supply is to an establishment outside of the UK and therefor the supply of the repair should be outside the scope of VAT. Is this the correct treatment? If so is the detail on the PO enough to document the establishment closes connected to the supply as the contract doesn't mention any of their establishments in detail? Also does it matter that the repaired goods stays in the UK for an undisclosed period of time before being used on the platform they say is closely connected with the supply?

Hi Winther01,
If you are providing a service of working on goods then this supply will come under the General Rule when determining the place of supply.
If your customer is in the UK then the place of belonging, of the customer, determines the place of the supply.
The place of supply would, in your case, be the UK and hence UK VAT would normally be charged.
Please see the guidance at Section 15. Zero rating and specifically section 9.7: 
Place of supply of services (VAT Notice 741A)
However, if you are working on goods which you then send overseas you can Zero rate the supply as per the guidance at Section 15 and specifically section 15.9:
Place of supply of services (VAT Notice 741A)
If the goods are exported then you will need this export evidence in your records to support the zero rating of these supplies
Thank you