Long advance notice- Rail Engineering Work is planned in South Wales from 3 to 18 July, which will affect all trains running through the Severn Tunnel between England and Wales.
Details will be tightened up much closer to the time- this is exceptionally (almost unheard of) long public notice of such work.
It is not even on the National Rail website yet- this is from Great Western.
Over some part of the line there will probably be bus replacements- whether this is east or west of Newport is not clear.
If east of Newport it will probably affect anyone on the Bath to Conwy journey. if it does it may be prudent to divert via London and Crewe or via Birmingham for that sector (using the through Birmingham to North Wales trains)