I have an inner pitch first tent, a Marmot Pulsar One. I have no real problems when it rains. I use my tent footprint to secure the tent poles then I throw over and fix the outer tent in the same way as normal. Once done, I just get under the outer tent with my inner tent and secure it to the poles. A bit of a wiggle but its fairly easy to do. If you haven't a tent footprint, just measure a couple of strings with a loop in each end and use these to support and tension the tent poles.
I enjoyed Offa's Dyke, especialy from Newport to Welshpool and found no problems in camping. I remember one in particulat, in Knighton. Its about half a mile from the train station, just walk straight ahead from the station, the rail line stays on your left as you walk along a narrow tarmac road. The camp site is a small field next to a farm on the left. I was there last year, £5 for the night, very nice lady, the grass was lush, like sleeping on a soft mattress. Then just a short stroll into the village, either by retracing your steps back to the railway station and then turn right, the first pub on your right is a good one. Or from the small field, cross the railway line, follow the footpath alongside the small river and you then come out onto the road at the Council Offices, turn left into the village.
Have fun and I hope the weather is good. Knighton is also the southern start for Glyndyrs Way. If you really wanted to make your walk even longer, hike this route, it eventually finishes at Welshpool and then you can rejoins the Offa's Dyke path.
