[Icare9]

You are correct in that a JH Stephenson was KIA on Stirling DJ977. As there is no further trace, it must be assumed that they were lost in the North Sea possibly on the return.
Tony Woods Combat Claims and Casualty Lists show 4 Stirlings claimed by nightfighters, although it is possible that a flak or fighter intercept didn't down this Stirling straight away, but it gradually lost height and crashed into the sea. Ordinarily, in that type of situation the crew might parachute or take their chance on ditching and getting into the life raft. Sadly that doesn't seem to have been the case here.
The Bomber Command Report for the raid is as follows:
19/20 May 1942 Mannheim
197 aircraft - 105 Wellingtons, 31 Stirlings, 29 Halifaxes, 15 Hampdens, 13 Lancasters, 4 Manchesters. 11 aircraft - 4 Halifaxes, 4 Stirlings, 3 Wellingtons - lost.
155 aircraft reported hitting Mannheim but most of their bombing photographs showed forests or open country. A concentrated group of about 600 incendiaries in the harbour area on the Rhine burnt out 4 small industrial concerns. Only light damage was caused elsewhere. The only fatal casualties were 2 firemen.
Hope that's a starter, then try 218 squadron website (if one) or Google crew names and/or aircraft ID, which might get you more!!