This particular book was published in 1952 and written by Walter Tyrer, a man who wrote a fair number of Sexton Blake adventures. I think the name is likely a pen name but I cant find anything out about the writer on the internet other than a list of titles he penned.
There's a rather good blog HERE that reviews many of the Sexton Blake adventures but the blog doesn't seem to be updated on a regular basis - pity.
Together with the Sexton Blake book I also picked up a bunch of old paperbacks as well as several issues of Warlord Comic. I do love browsing in secondhand books shops and Cardiff's Troutmark Books is an excellent store. I'd recommend a visit to anyone who finds themselves in Cardiff. You'll find it in the Castle Arcade which is across the road from Cardiff Castle.