Some googling and some general questions in the office confirm that the monster pair of insects that were in the garden yesterday are indeed stag beetles.
I would have liked to go fetch the decent camera, but I was already a little late for work, so phone camera will have to do. The things must have plenty of food in the garden, keys are for size compare.
DT.
Thats an evil-looking bugger!
At first I thought it looked scary too. But just reading up on it…
…if you can find them again get your better camera 
They don’t live long in beetle form, You’ll only see them between May to August about looking for a lady friend.
P.s. The Male’s can fly…
edit I think they may be near to being an in dangered species as well ??? there was something I heard about a Bio-diversity study being done…
/edit2
I’m going to go looking for them again tonight, I think you can make out just underneath one of the beetles that there could well have been a female there as well, but someones foot got there first 
Going to have words with the kids tonight, as they are on the decline and where I found them is a perfect habitat for them from the various links I followed.
DT.
neighbours kids have killed one of them remains are where I found them yesterday 
I’m hoping that there are some more living in where I put all the dead wood and bits late last year.
/edit - we’ve been to speak to them and they killed the two I saw yesterday
DT.
found a female one today 
not as impressive as the males, but hoping it will not be long before she attracts the impressive looking males.
DT.