Thanks, this is an excellent workaround. On my system it requires a two-stage process to get a good result. First, cut off the start off the file (using a file splitter, not avidemux) as you suggest so that it starts with all 6 channels playing. Now recode the audio in avidemux and downmix to 2- channel AC3 (or whatever your preference) without making any cuts whatsoever in avidemux (i.e. leave all the adverts in). There is no need to recode the video at stage 1. Then take the result, load into avidemux, cut out the adverts and recode the video. This produces a good result. If you try to cut out the adverts at the first stage, the re-coding of the audio fails after the first cut point, more often than not. I think this is at least in part because the broadcasters do not make the switch from 6 to 2 channel and back again on the exact same frame as the video changes.
As a two stage process it’s not quick, but it does work and produces good results, apart from tiny audio blemishes when the channels change, which is probably unavoidable.
Also, for windows folks like myself, I have not been able to find a tool that cuts a given number of bytes off the start of a file. HJSpit carves the file up into equal sized chunks. H264 TS Cutter is an alternative that seems to be good enough to get the job done even though it might be altering the stream slightly. I have also tried splitting with MKVtoolnix, but for some reason this does not give the right result. Any other windows file cutter suggestions welcome – ideally just want to slice a file in two, but to choose unequal sizes for the two parts.
Despite these caveats, all in all it’s a fantastic workaround, many thanks.