If only bi-mode had been an option back then. That said, could you not have put a 43 on one end of the formation permanently?
Problem with bi-mode is you really need to be able to shut down the diesel unit, and a large diesel unit e.g an HST power unit is very heavy, so you'd have impacted the performance of the overall set. According to Wikipedia a DVT weighs circa 40 tonnes compared to an HST power car at 70 tonnes. That, or you run with a diesel engine active under the wires - which seems to attract the ire of a number of posters round here.
The HST power cars used as DVTs were only ever a temporary measure whilst the genuine DVTs were built - and it probably worked as the 91 sets were replacing the HST turns on the ECML. I believe it was always planned that the HSTs made redundant from the ECML were to be cascaded to allow other, older stock to be withdrawn.
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Worst one for me a 153 on a peak service between Northampton and Shrewsbury was cancelled at Birmingham International due to overcrowding
But that had to be a one-off?
I've never seen a 153 out of Northampton, and DMUs only appear in service around Northampton when there are major problems - I think the last time was when the whole of the 321 fleet was grounded about 5 years ago - which led to no end of strange workings.