I appreciate this is a bit late however the following might be of interest.

The Fort Record Book for Devils Gap Battery contains a brief history of the battery which contains information about two incidents in WW1, including the one mentioned by islandman. The first incident is described thus:

"On 4 August Great Britain declared way on Germany. No hostile action was attempted by enemy forces till 31 Dec 1915. On this date, at about 2200 hrs enemy submarines attempted to enter the bay. They were detected by Haynes Cave Battery which opened fire. Some of the other batteries joined in later. The action was over by 0030 hrs. Some or all of the submarines are claimed to have been sunk."

The second incident is recorded thus:

"on the night of 2 June 1917 at about 2300 hrs Devil's Gap opened fire on a submarine lying close inshore to Algeciras. The submarine is supposed to be sunk. Several shells from DG or
??? Battery ricochetted onto the cemetery at Algeciras, in consequence of which DG and other batteries were refitted with Half Charge. After a short time however they were refitted with Full Charge shell."

It is notable that both these incidents took place late at night and that the submarines were either "claimed to have been sunk" per 1915 and supposed to be sunk" in the case of the 1917
incident. There is no actual statement with supporting evidence that says they definitely sunk. I addition, the history appears to have been written around the time of the battery becoming non-operational in 1952.

I suspect that the two incidents were cases of shadows in the night and over excitement by the gun crews.