Rough summary, from the book British Railways Mk1 Coaches, by Keith Parkin, published in 1991.
In 1984 a rake of 13 mk1s was refurbished at Derby to show what could be done with them if a bit of money was spent. New ceilings, lights, seat covers, carpets and toilets, including a corridor second coach, with six-seat and eight-seat layouts in the compartments. Set was painted in InterCity executive livery. It was passed to run at 100mph and, as 455driver says, was used on Derby/Nottingham-St Pancras services for a while - I think with 37/4s as traction - then went into the BR charter fleet.
Some of the ideas from the refurbishments were adopted in other Mk1 stock that was expected to run for a few more years, especially catering cars, but also Mk1 first class stock used on Motorail trains and charters. All of these got the executive, then Swallow livery. GUVs used by Motorail also ran in this livery.