(http://www.argos.co.uk/wcsstore/argos/images/198-5531062A72UC576004M.jpg)
We have started using a specific set of walkie talkies that we feel provides a decent performance at a reasonable cost. They are Binatone Action 950s which have 8 channels, dual watch, operate at up to 10km and have a backlit display. On top of this they look fairly professional and to date have worked well for distance and battery power and have given an excellent call quality. Obviously they work on line of sight so in the mountains reception can be patchy but we have had them work from Slieve Donard Summit as far as Hares Gap. At the time or writing they are £44.89 from Argos.
These are PMR446 standard walkie talkies so they should work interchangeably with any other PMR446 units.
We use channel 6 so if you are coming to a meet and have one of these units then you should be able to contact us once you are within range. If possible let us know in advance so we know to keep them switched on.
You can see a detailed review here:
http://www.qsouk.com/pmr-f7/binatone-950-review-t1242.htm
You can buy a set here:
http://www.argos.co.uk/static/Product/partNumber/5531062.htm
Update January 2014
The 950s are no longer stocked by Argos. This is a link to the 1000 series. I can see no reason why they would be any lesser quality (they have 60 reviews and an average of 4.5 stars on Amazon) and they should communicate with the 950s as the public band is shared and the same for all units. We'll try and get a test sorted as soon as poss.
http://www.argos.co.uk/static/Product/partNumber/5531103.htm
http://www.amazon.co.uk/Binatone-Action-1000-Two-Way-Radio/dp/B008FOBL1A/
Update August 2016.
Is now the Action 1100s
These still review well, however, the now list more channels. As I understand it, the additional channels are "side channels" or "sideband" for you 80s CB radio junkies. They should still work with the older models and channel 6 should be fine but we would need to lay our hands on a set to test to be sure and to clarify. If anyone can clarify reply beneath.
http://www.argos.co.uk/static/Search/searchTerm/action+1100.htm
https://www.amazon.co.uk/Binatone-Action-1100-Two-Way-Radio/dp/B00GZHD0RM/
At time of writing, Amazon are significantly cheaper than Argos.
(We have no affiliation with Argos, they are just easy to use since they have plenty of outlets but the units are available from a variety of stores.)
Please note also that there are likely a whole variety of walkie talkies types that will be capable of communication on the channel we use so if you already have walkie talkies then they may be suitable.
In addition, I know from experience that other models of Walkie Talkie will have the same channel 6 as these, I've even used kids ones and they have worked fine. Testing would be essential first though.
Feel free to comment below with advice on how to get other radios to communicate on the same channel or with any other useful information.