Does it have a makers name? If it is a well known name you could see if a website such as http://www.webuyyourwatch.co.uk/ was any help.
Also an independent high street jeweller may have a valuer/appraiser who could help.
The British Horological Institute may also have some suggestions. They have a page on "find a repairer" repairers can often give a ball park figure for items. http://www.bhi.co.uk/
The National Association of Goldsmiths runs a valuers scheme and many of them would deal with watches. http://www.jewelleryvaluers.org/
Stephen - You could try Malcolm McQueen - McQueens jewellers in Dungannon. They used to have a jewellers shop in Magowan buildings in town as well but Malcolm has been scaling back work....