Hi Roger,
The first 24.5t hoppers were built in 1955 at Shildon to pattern 1/148. They had LNER-style clasp brakes and oil axle boxes. Our version is the revised 1/148 pattern introduced from 1958, which featured a handbrake system with offset v-hanger and longer brake handle, as well as having roller bearing axle boxes. This version made up the vast majority of 24.5t hoppers built, with 3,426 built at Shildon between 1958 and 1962 (I'm including the 1/154 pattern in this figure because they're the same design - having consulted with a number of people who've conducted thorough research of BR wagons, it appears the assignment of a different pattern number was a clerical error! Having inspected the surviving 1/154, it matches everything we have on the 1/148s...).
As you say, they were assigned to wagon pools supplying coal to power stations (High Marnham, Stella North/South Dunston/Blyth, Kincardine, Staythorpe, and Thorpe Marsh initially), and could be observed in block trains or mixed with 21t hoppers and the larger mineral opens. BR later abandoned the idea of having them assigned to specific flows and blanked out the route info on the body sides. They had the opportunity to 'stretch their legs' a bit further from this point onwards. As well as power station flows, they could be found in domestic coal and coke traffic, as well as carrying stone (from Wirksworth in Derbyshire, for example). The latest photo I've seen of them in traffic is of a coal train in Wales, which dates from 1987. The complete pattern list for the HUO fleet: 1/148 (early) 1/148 (later)
1/153 (as 1/148 but initially fitted with vacuum braking and retained some of this equipment after it was deactivated) 1/154 (the clerical error - 1/148s in reality) 1/155 (revised hopper design) Hope that helps!