ATW usually have "pick out of the sky" 150s available for additional work. Also Maintenance schedules can be worked round to provide additional capacity.
An additional 158 has been found here (probably come from the South and replaced by 150 but not sure of this). The 158s involve the Holyhead to Birmingham Ints, the normal working at Shrewsbury for those who may not know:-
Birmingham Int to Holyhead arrive Shrewsbury in Platform 4 as 2x158 (3 on one service) and detach the rear unit which remains in Platform. Some minutes later Holyhead to Birmingham Int (1x158) arrives and attaches to the Unit left in Platform 4 to go forward to Birmingham as 2x158. However, during the occupation the train to Holyhead leaves a couple of minutes later and the train from Holyhead arrives a couple of minutes earlier which means the normal move cannot take place due to the time factor. So, our additional 158 works the Shrewsbury to Crewe locals (vice 153) and the timings of these fit in just nicely.
Train from Crewe arrives XX11 in Platform 7 and the Holyhead to Birmingham arrives and attaches to the rear of it departing from Platform 7 at XX33 as usual. The Birmingham Int to Holyhead kicks off the ear unit as usual in Platform 4 and then works the 14 33 local back to Crewe.