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Freddy - I have just seen your correction in post #109 re 'iron' gas pipe. You are of course correct that the fittings are cast iron, either galvanised (essentially for outdoor/damp conditions) or 'black iron' finish, which is I think the result of a chemical process (oxidation?) during the casting process and gives a certain amount of natural corrosion protection. As to the pipe, it is not cast iron (at least in small supply pipe sizes) as this is far too brittle. Although often referred to as 'malleable iron' (meaning it can be bent), it is I think usually/normally drawn mild steel these days and is again in galvanised or black finish. Such suppliers as I have identified do say that it is steel. I don't think that corrosion is a real problem in iron/steel gas pipes/fittings, given their wall thickness. As you say, iron resists rusting better than steel. Interestingly, the only gas leak I have ever experienced was from a 'black steel' pipe inside my house. It had been installed only about 5 years earlier by British Gas to supply the meter, which was in the bottom of a cupboard but quite close to (damp) earth below the floor. I was not impressed and insisted on a repair FOC, using galvanised. In those days (1978), if not now, it was usual to run gas in steel pipe wherever this was not an eyesore (when copper was used). Better for resisting damage but not necessarily corrosion. A further thought regarding your re-plumb in copper. You may well find that your local scrap merchant sells brand-new copper pipe (and fittings) at a good discount. Mine certainly does. I don't know where it comes from - although obviously 'leftovers' from a job somewhere - and don't ask! Although the pipe is invariably straight, clean and undamaged, it is of course worth checking, especially if the bundle has been made up from different sources. The fittings (usually there by the bucket-load) are particularly cheap as sold by weight - a handful costs almost nothing. Until recently, I too would not touch plastic pipe. But I have since used it (although not for gas!) and am quite impressed - although I avoid joints in visible locations. One location where (white) plastic 15mm pipe is handy is in the upstands from below the floor into the rad valves - no painting necessary! If necessary, they are joined to the (horizontal) underfloor supply pipe with either a plastic or a brass compression elbow.

The noises you describe ('knocking') can also be caused within the boiler itself as key parts expand, although probably not so much in a combi which is of course designed to be 'off' except when heat is called for. Yes, pipe expansion/rubbing will also cause noise. And if the noise is not eliminated in a short time after firing-up (or comes back), the pump may be running at the wrong speed in a pumped system. I bought some felt pipe lagging (a sewn tube) a couple of years ago and it is still available. As you suggest, just the job for packing around a pipe which is running through notches or past rubbing obstructions. One can use ordinary loft insulation too - pushed into the gaps around pipe - or even stiff paper or polythene; anything to pad out the gap without adding to friction. I agree that too much 'float' is to be avoided; pipes should be well supported, even if it is a resilient support. Unless you exert too much pressure when using a pipe cutter, the pipe should not distort, although the internal burr is best removed with a suitable tool as this will reduce bore slightly and impede flow. A slight dent or ovality is of no importance; a properly cleaned end-feed soldered joint will seal adequately and the fitting will usually help to correct distortion. If using solder-ring fittings (which I generally avoid), some extra solder is advisable if any question of a loose fit arises. The key to a good joint is cleanliness/enough heat/adequate flux. And a 'wipe round' with a slightly damp rag after making the joint should help if any solder does not flow as it should. Take your time - correcting dry joints can be a bugger after water has been introduced!