Hi Martin
Here is the debug for Colossus running on Arena. I am not really that familiar with UCI protocol, maybe you can see something wrong with Arena's output ?
1554500>1:isready
1554532<1:readyok
1554610>1:ucinewgame
1554610>1:isready
1554672<1:readyok
1554719>1:position fen 5K2/8/2qk4/2nPp3/3r4/6B1/B7/3R4 w - e6 0 1
1554719>1:go infinite
1554750<1:info depth 1 seldepth 1
1554766<1:info currmovenumber 25
1554766<1:info depth 1 currmove d5c6 currmovenumber 1 nodes 0 time 1
1554766<1:info depth 1 score cp -174 pv d5c6 d4d1
1554766<1:info depth 1 currmove g3e5 currmovenumber 2 nodes 4 time 1
1554766<1:info depth 1 score cp -114 pv d1d4
1554766<1:info depth 1 currmove a2b3 currmovenumber 4 nodes 18 time 1
1554766<1:info depth 1 currmove f8f7 currmovenumber 23 nodes 168 time 17
1554766<1:info depth 1 nps 10352 nodes 176 time 17
1554766<1:info depth 2 seldepth 7
1554766<1:info depth 2 currmove d1d4 currmovenumber 1 nodes 176 time 17
1554782<1:info score cp -160 upperbound
1554782<1:info depth 2 currmove d5c6 currmovenumber 1 nodes 290 time 17
1554782<1:info depth 2 score cp -174 pv d5c6 d4d1
1554782<1:info depth 2 currmove g3e5 currmovenumber 2 nodes 315 time 17
1554782<1:info depth 2 currmove g3f4 currmovenumber 23 nodes 361 time 17
1554782<1:info depth 2 nps 21294 nodes 362 time 17
1554782<1:info depth 3 seldepth 7
1554782<1:info depth 3 currmove d5c6 currmovenumber 1 nodes 362 time 17
1554782<1:info depth 3 score cp -160 pv d5c6 d4d1 a2c4
1554782<1:info depth 3 currmove g3e5 currmovenumber 2 nodes 591 time 17
1554782<1:info depth 3 score cp 94 lowerbound pv g3e5
1554797<1:info depth 3 currmove g3e5 currmovenumber 1 nodes 652 time 17
1554797<1:info depth 3 score cp -2 pv g3e5 d6e5 d1e1 e5d6 d5c6 d6c6
1554797<1:info depth 3 currmove d5c6 currmovenumber 2 nodes 845 time 17
1554797<1:info depth 3 currmove g3f4 currmovenumber 23 nodes 1014 time 17
1554797<1:info depth 3 nps 60000 nodes 1020 time 17
1554797<1:info depth 4 seldepth 11
1554797<1:info depth 4 currmove g3e5 currmovenumber 1 nodes 1020 time 17
1554797<1:info depth 4 score cp -2 pv g3e5 d6e5 d1e1 e5d6 d5c6 d6c6
1554797<1:info depth 4 currmove d5c6 currmovenumber 2 nodes 1569 time 33
1554797<1:info depth 4 currmove g3f4 currmovenumber 23 nodes 1849 time 33
1554797<1:info depth 4 nps 56090 nodes 1851 time 33
1554797<1:info depth 5 seldepth 12
1554813<1:info depth 5 currmove g3e5 currmovenumber 1 nodes 1851 time 33
1554813<1:info depth 5 score cp -10 pv g3e5 d6e5 d1e1 e5d6 d5c6 d4f4 a2f7 d6c6
1554813<1:info depth 5 currmove d5c6 currmovenumber 2 nodes 4076 time 64
1554813<1:info depth 5 currmove g3f4 currmovenumber 23 nodes 4356 time 64
1554813<1:info depth 5 nps 68125 nodes 4360 time 64
1554813<1:info depth 6 seldepth 14
1554828<1:info depth 6 currmove g3e5 currmovenumber 1 nodes 4360 time 64
1554828<1:info depth 6 score cp 16 lowerbound pv g3e5
1554828<1:info depth 6 currmove g3e5 currmovenumber 1 nodes 8024 time 64
1554828<1:info depth 6 score cp 16 pv g3e5 d6e5 d1e1 e5d6 d5c6 d4f4 f8e8 d6c6 a2d5 c6c7
As you can see, Colossus is missing the mate, but you can see Arena has told it the ep square is e6.
Operator bug ?, Arena Bug ?, Chess Engine bug ?, jury is still out I reckon !
Hope it is not an operator bug, but wouldnt be the first time
Geoff