Subsidence on front porch extension?

SAFCDan

I've recently bought and moved into my first house (start of June), and there appears to be some subsidence at the front of the porch extension, which was built roughly 20 years ago (The house itself is 30 years old). I can't say for certain that it happened recently, but can't say that we've really noticed it until now!

For what it's worth it wasn't picked up in the Home Buyers Report so it's possible the recent hot and dry spell hasn't helped

Some pictures -be warned, they're quite large! Just to give an idea of size, the gap is large enough to get a hand in.

i.imgur.com/Hpw66v0.jpg

i.imgur.com/Ow6T4C3.jpg

I'd appreciate any advice or suggestions, should I be rushing out to get a building surveyor out? :undecided


[edit : Just to confirm the "sinking" bricks are only the outermost layer of the extension, having had a look inside the crack anything beyond the outermost layer of bricks seem fine]