How old is the horn? Any other problems or damage? Was it purchased from a UK retailer?

Its not difficult to take horn valves apart if you know what you are doing but putting them back together is more complicated. Furthermore if you have a valve which is poorly made then correcting that can be a specialist job which is expensive.

Most horn valves are made to a very fine tolerance and even reputable makes such as Holton and other professional instruments can have stuck valves.

If you can't find a good repair shop in Liverpool or Manchester (I don't know those cities very well) you could try posting on http://www.horncups.com or look at http://www.harrogate.co.uk/harrogate-band/link08.htm Unfortunately, too many repair technicians wont look at Chinese made horns unless they have a well known brand name on them. IMHO if the valves did work once then they can be made to work again in less than 1 hour. If they never worked then your safest option is to sell the horn on ebay - with a honest and accurate description of the horn.

John Myatt Woodwind & Brass is a respectable firm (I should have guessed that JM=John Myatt!) I assume that the valves worked properly when you had the horn delivered. If you ask them, they'll provide an estimate of repair costs for the valves - although shipping both ways may be more than the cost making the valves work. They could probably take the dent out but that would be more expensive. If the dent is in the throat to the bell then unless it closes more than half of the airway its unlikely to be critical when playing.