Can we have some pictures of the pipwork etc in the Airing cupboard please for clarification? If you do have an air seperator, (the metal can I spoke about), these are notorious for blocking. Whilst the pipework either side may well still get hot, if the flow of water past the blockage is poor, then it will lead to the circulation issues you have described.
X400 will not dissolve a solid blockage, the only solution there is to cut out the offending section and either throroughly clean it mechanically, or usual practice is to just replace it.
X400 will not dissolve a solid blockage, the only solution there is to cut out the offending section and either throroughly clean it mechanically, or usual practice is to just replace it.