Matching lawn seed

Hi all - I have a patch in my lawn (around 3x3m) that never drains, and after some digging it seems that the soil there is really clay heavy. My plan is then to remove the clay, add some gravel etc for drainage and then top with good soil before covering with new lawn.

I've taken up the turf and kept to one side hoping to relay, but it's so embedded with the clay that my concern is the surface water will still remain on top. I'd therefore like to relay some grass seed (or new turf) but wondered if there's an easy way to identity what lawn I have now so I can match it with what I buy. The last thing I want is a square of completely different lawn in the middle of my garden!

Thanks