does raise a question, if pacers only need be withdrawn for DDA requirements, how about only ever using them to strengthen other services operated by 150, 153, 155 or 156 units that will be DDA complient?
Well firstly, 142s will be life expired before DDA kicks in so the DDA compliance is not the only issue.
However, in answer to your question I think it would depend on the length of the train. It has already been declared that a non-complaint HST can not have a single complaint vehicle added to it to make it complaint. However, it may make a difference to have a 4 car unit where half of it is complaint but if it was you wouldn't be able to lock the complaint train out-of-use for part of the journey because platform lengths.
The 156s and 158s are set to undergo alterations to make them compliant over the next few years. The 153s are to be reviewed as being single carriage trains they may prove too expensive to make complaint in their current form but possibly reforming them in to 155s could solve that problem.
It's been all quiet on the 150 DDA compliance issue. I imagine the former FNW 150s (which have been made partially complaint already) will have new door controls and internal and external LED destination displays added to them and be allowed to continue in service but the non refurbished ones may prove too costly to do given their likely expiry date.