I have done similar on 2 occasions, one a woman was in Boots in a squat position with an Easter in both hands, her purse was under her arm, A guy grabbed the purse with such force it spun her around and onto her back, I acted without thought and gave chase, after a couple of hundred yards I realised how big he was but almost had him so no stopping, I tripped him and bundled him down with me on top, one knee on his chest and the other in his throat, luckily we had passed a police woman during the chase, she approached and I let him up, as she was cuffing him he pulled away and ran, I followed and took him down a second time, by then there was police everywhere, I ended up in court as a witness and got "highly commended by the judge"
The second occasion was, I was passing a jewlers in the town centre and all hell broke loose, there was banging and crashing and the window displays where getting thrown all over or so it appeared, by the time I realised what was going on 2 guys came running out of the shop, again without thought I dropped my bags and chased, they split and I followed one down an ally, the problem arose when we got so far down it was gated, thats when I thought oh s*** and he threatened me with a knife, I should have walked away but I just had to follow through, I took him down and got all the stolen gear off him, looking back this was probably the most stupid thing I have done