Green’s Dictionary of Slang

1. a cake.

1927		Hartlepool Northern Dly Mail 28 Jan. 5/5: I heard a coffee stall customer ask [...] for a ‘Once or twice of Sexton Blake, please’ and the proprietor said, ‘Will you have it in your German or the linen draper?’.
1960		J. Franklyn Dict. Rhy. Sl.
1971		J. Jones Rhy. Cockney Sl.
1984		V. Darroch On Coast 77: Sexton Blake: Sunday cake.

2. a fake.

1979		F. Norman Too Many Crooks Spoil the Caper 170: It sure don’t look like a Sexton Blake.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.
2007		N. ‘Razor’ Smith Raiders 79: The hot-watch man produced a jeweller’s loupe [...] ‘Well, what you have here, my friend, are a pile of Sexton Blakes [...] fakes.’.