Thank you all for your suggestions. You've been really helpful.
I did a number of things since this post and I haven't had this problem since! Only thing is that I'm not sure which one, or all things worked.
Here's what I did;
-Higher octane fuel fill ups on the next couple of tanks
-Changed the spark plugs
-Cleaned the MAF sensor
-Replaced the O2 sensor
-Replaced the starter (probably not relevant but I had to anyway)
-Manually choked on a start (may have helped with calibrating the air/fuel ratio)
I still need to fix the exhaust manifold pipe which may help with gas mileage, but the lagging is gone!