Stag beetle grubs

I was digging out a very rough area at the back of my plot a couple of days ago - full of couch grass and stones.  During the process I dug up a couple of large white grubs with light brown thoraxes and heads.  My first impulse was that they were cockchafer grubs but a closer look showed that the mouth parts were side moving rather than vertical.  So, my revised thought is that they were larvae of the stag beetle.  Carefully buried them back at the same depth I found them.

The grubs live in rotting wood laying on the ground, so if in open ground probably not. I was clearing out an old dalek bin and found loads of white curled up grubs in it, also found some dormant irredescent beetles so assume that's the source.