I adapted a small (1/4" drive) socket set to clean up the injector seats... I stuffed a small wad of kitchen roll into a 10mm socket on the end of a 120mm extension shaft and fitted a hand drive (so like a screwdriver), dipped the wad into some petrol, then twisted it down into the recess to clean up the mess and allow me to shine a torch down and inspect the seat area. Didn't look too bad, but there was some persistent grime, so swapped the kitchen roll wad for some medium grade steel wool taped to another socket and smeared with grease. Twisted this down onto the seat and worked it back and forth for a while, then put the petrol-soaked socket back on and cleaned up the grease and fresh debris. This technique worked well and left the seats looking in a good clean and shiny condition. Fitted new copper gaskets (from Holdcroft Honda) and put the injectors back in, having cleaned up the gasket seating surfaces. Took me about 45 mins per injector, including cleaning and refitting all the high-pressure pipework. The only problem I had was tightening #1 injector clamp bolt, as oil had filled the threaded hole and the hydraulic pressure resulted in me wrongly thinking I'd correctly tightened the clamp when I hadn't (the injector was still very slightly loose even after torquing up and turning the clamp bolt a further 90 degrees). I had to remove the injector and clamp to clear the hole with cotton buds... should have checked this first!!!
I only idled the engine for around 20 mins plus a short test drive after refitting the injectors, but there are no obvious leaks from the injector seals.