Finally back, so thought I'd update the thread in case anyone else searches for the DEL lounges.

The two lounges in T3 for transit pax are actually quite pleasant, not great but not a bad place to spend a few hours: On the way out I used the upstairs lounge (A), which is small and rather unremarkable. Nice enough though.
On the way back I used the much larger B lounge on the ground floor. Plenty of seats, a decent bar with snacks+3 choices of warm food and the possibility for a shower. The only mice to be seen were all attached to laptops.

One word of warning, though: The lounge 'guards' seemed rather confused by someone with a BAEC card wanting lounge access, their rules only catered for CW+F fliers. To save headaches if you're flying in WT or WTP it might be worth printing out BA's access rules (or the MMB section that says you have access) to show the guard.