Porch Light - changing the bulb

[YSK]

Hi all - I'm looking for some advice on how to change the light bulb in my external porch light.
Apologies if this is too basic a question for these forums or if I'm in the wrong forum - if so, please advise where else I might try.
I am unable to access the light bulb to change it. I tried to unscrew the bolt at the bottom of the fitting - this came out but had no effect other than allowing me to rotate the glass cover, and now I can't get the bolt to screw back in. I have tried turning the bottom half of the unit to the left, but it wouldn't budge.
I am attaching some photos which, hopefully, will demonstrate the type of fitting I have.
Any advice greatly appreciated.
Best wishes
YSK

 

[333rocky333]

At a quess the bottom frame twists of the top frame, check carefully sometimes theres a small Allen key Grubscrew to lock it on, often located out of sight. maybe to the back

 

[Sureitsoff?]

might be easier to take it off the wall and work on a bench to see if there are any hidden fixings

 

[mattylad]

 

[crystal ball]

Reminds me of that old joke