You have to go back to 1987. That year, the Maintenance Depot at St. Leonards closed, with the remaining Diesel units being transferred and maintained at Selhurst.
Three units were required to operate the Marshlink services back then, and there was a late evening/overnight working which would see one unit return to Selhurst and be exchanged for another if required. At that time, the other units were still programmed a daily visit Chart Leacon Depot at Ashford for fuel.
After Chart Leacon ceased fuelling, the units then had to visit Dover Town Yard to refuel, making things more difficult.
Things got more complex with the 'route branding' in later NSE days.
There were a number of units that were provisionally set aside with 'Marshlink' stickers, at one time these were 205101, 207101-3, 203001. When 203001 was withdrawn, 205016 was rebranded for Marshlink use, while the remaining Selhurst Units gained 'Oxted Line' stickers for the Uckfield Line. However, it nearly always seemed impossible to supply three serviceable units for the Marshlink services, and occasionally it meant loaning 'Oxted' branded units to cover.
Internal politics took over at some point and South Central put a block on sending any 'Oxted' branded units to cover the Marshlink services, leading to a number of occasions when the additional Peak Hour services between Hastings and Ashford were cancelled. (Strangely, it didn't prevent 'Marshlink' branded units turning up occasionally on the Uckfield services).
Being based at Hastings at this time, I well remember all the problems we had back then. Very early on there was still some flexibility in the late 1980s and there was a short spell when a DMMU was brought in to cover the shortages off the Tonbridge to Reading service. It was manned by Tonbridge Crews, and worked down empty from Tonbridge, did two trips across to Ashford, then on the last trip went into Chart Leacon to fuel, and then went empty to Tonbridge to work an afternoon service to Reading.
By the 1990s, this form of flexibility finally died. I have some pictures of the DMMU workings from a weekend when it was used to cover Ashford to Rye services during Engineering Works.
Much of the 'difficulties' were resolved when the Marshlink services were transferred from South Eastern to South Central control.
After privatisation, South Eastern Hastings crews were still used to cover some of the workings, but with the advent of the new 170s the work was all moved to South Central crews based at Eastbourne.
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Turning round in the Down platform at Rye.
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Running into Appledore from Ashford.
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When 203001 was in use, shunting out of the Cleaning Shed at St. Leonards. Marshlink branded sticker between the nameplate and coach roundel.