This mirrors the experience my relatives had on an otherwise very well planned trip. My mildly picky sister actually refused to shower there .

At the prices posted by Ozchinois for the Pan Pacific, it seems like a no-brainer to opt for this hotel which is considerably nicer than the Concorde Inn.

The Plaza Premium lounge isn't worth paying for, unless desperate for a shower and beer on tap: the food options are sparse, the computers few and slow (admittedly a few years ago, but they were running Windows 95 IIRC), drinks and showers ok (although the dressing area of the stall gets flooded).
The MH lounges are in another league, but off-limits to all but MH premium passengers now (NW and KL Golds and above used to be admitted).
There is abundant free seating at the underused international gates in the main building (not the satellite), although beware the psycho toilet cleaner who resents people using 'his' facility

Landside, there is a vast foodcourt downstairs on Level 2, with vastly more attractive prices and selections than the mediocre airside eateries.

However, with presumably long flights, on which there is little to do but sleep, before and after the layover, a quick visit to Kuala Lumpur proper would be the way to go. Quick immigration and emigration, 35mn nonstop train ride to downtown, great eating at bargain prices, tropical climate for a contrast with the UK... the main downside to KUL the airport is slow luggage delivery, which for transit passengers is not a concern.