Hello David,
I had not forgotton you but had to do some further research to answer your question. I am pretty sure he was wounded 23rd April from various circumstansial evidence. Harold appearing on a casualty list would have been a definite clincher. Allan Parry has done the Worcester Herald but there are others you could look up.
1. Harold only had a 1 in 8 chance of surviving the night of 23rd April uninjured.
2. You said he laid in the mud all night, so it had to be a night action.
3. On average it took 6 months to get discharged after a wounding. Harold was discharged in Oct 1917.
That is where the extra research has come in. I had to find some men with injuries so severe they had to be discharged. The fastest discharge I found was 3 months for a man with a leg amputation.
Pte Brown 6510 had a leg amputated April 1915 and was discharged Jan 1916. Pte Hemming had severe wounds Nov 1915, discharged June 1916. Pte Smith 7500 severe wounds May 1915, discharged Oct 1915. The army were not quick to discharge. Communication was very slow then, everything done by letter. The army also did all it could to get a soldier back to the front. They did not want him going home.
Hope this answers your question Mike