Sealing the top is very simply. I cut out a top from another piece of pipe and siliconed is on to the top. That way I could have easy access to the main guts during more serious maintenance. It was actually pretty easy to get on and off and the whole thing stayed dry past 10 feet of water.
Dont get me wrong the endcaps worked really well. I used a pair of channel lock plier to tighten the caps down for a run and they never let a single drop of water in. It just didnt enjoy having to use a pair of pliers to get the thing open. I also didnt leave enough room between the battery and the servo glued to the cap. This would cause the wires to tangle sometimes.
AS for source of materials. Im not sure if McMaster-Carr will do the UK but its worth a try. Pvc is always cheap and so easy to turn. Im using deralin because its stronger and hydroscopic. It is also a bit pricey. O-rings are always a pain to find yet alone figure out which size you need. Im luck in that being a submarine mechanic I have access to all sorts of o-ring sizes.