As an update on my original post, I ended up buying a DT125 for £750, spent about a ton on a few basic spares (sprockets, chain, reg/rec due to burntout original, bulbs) and a service, doing the work myself.
I went to Ireland in the end and did over 2000 miles in 10 days, riding from Peterborough down to Pembroke Docks in South Wales, across to Rosslare by ferry then along the south coast to Killarney and the ring of Kerry, back through the interior counties to Rosslare and home.
The bike didn't let me down apart from many blown bulbs and a fried battery (filled it back up with water and off I went again). It's not fast and not particuarly comfortable but it was certainly reliable.
Ireland is a beautiful place to ride around; loads of open spaces, inspiring views and a huge variety of road surfaces. The weather isn't great, especially as I camped the whole trip, when it was sometimes hard to dry out after a day of biking in the rain!
Now I'm planning on getting a bigger bike for my next trip but would definitely recommend the DT.