I'd say fairly stand-alone from memory. Each story has a very different plot, and sometimes a different setting too. His relationships with other characters do develop as the series progresses, but that's not enough to spoil things if you decide to go back and start from Book One afterwards IMO
I've wanted to read one of the Shardlake books for a while and tonight the library had the fourth in the series so I got it but just want to check whether I'll be ruining the series by reading fourth first or whether they are fairly stand-- alone. Thank you.