Here's what those nice people at Wiki say about the Class 48s:
https://en.wikipedia.org/wiki/British_Rail_Class_48
These are the relevant sections, although they don't say exactly why the experiment was discontinued:
"
In service
The locomotives, numbered in the D1702-D1706 series, mainly worked from Tinsley depot in Sheffield, on both passenger and freight work. In 1969, they moved to Norwich depot where they worked on express trains between there and London Liverpool Street. The 12LVA24 engine, however, was found to be unreliable, and the locomotives spent more time out of service than their standard counterparts. Engine failures were common, and repairs often expensive. After conversion to class 47 they moved to Stratford depot, in East London, until they were displaced by examples fitted with electric train heating equipment. Subsequently, the batch went their separate ways, to several depots.
Rebuilding
Eventually it was decided not to continue with the 12LVA24 experiment, and it was decided to remove the engines and fit the standard 12LDA28 engines to the locomotives. D1702 was the first to be so treated at Crewe Works, using parts from D1908, withdrawn after a serious accident. It emerged in December 1969. All five locomotives had been so converted by early 1971, and then became standard Class 47s. The power units were sold to SNCF and used in their Class A1AA1A 68000 locomotives."