What's this in my fireplace?

[SteadyRolling]

I've just removed the fire surround (for a modernish electric fire) in my 'new' Victorian house, in preparation for opening up the fireplace for a multi fuel stove. There are modern bricks behind it, which I half-expected to see, but at the bottom (see pic) there is a metallic-looking plate that I didn't. On its left hand side there's a disc with a slot, and the word Open with an arrow above it. I can turn the disc about 90 degrees but it doesn't seem to have any effect - so just thought I'd better find out what it's actually for before I begin demolishing it...

 

[lostinthelight]

View attachment 134213
I've just removed the fire surround (for a modernish electric fire) in my 'new' Victorian house, in preparation for opening up the fireplace for a multi fuel stove. There are modern bricks behind it, which I half-expected to see, but at the bottom (see pic) there is a metallic-looking plate that I didn't. On its left hand side there's a disc with a slot, and the word Open with an arrow above it. I can turn the disc about 90 degrees but it doesn't seem to have any effect - so just thought I'd better find out what it's actually for before I begin demolishing it...

looks like a soot box door to give access for cleaning the flue.
Turning the disc should turn a catch , try getting a blade in between the door and frame to lever it out.
litl
Edit
this is whats on the other side,there may also be a tin inner cover

 

[SteadyRolling]

Ah, thats great, thanks, will check in the morning. Wonder why they thought it necessary to install that ...