I found a Lea & Perrins sauce - more or less ketchup (presumably Heinz ketchup, that being the company making it) with some Worcestershire sauce added - which seemed to work quite nicely. Discontinued now I think, which is a shame.

In terms of actual ketchup, most of the other brands seem quite disappointing, though the Branston one seems quite good. I'm fairly sceptical of most "own-brand" products these days; a few work fine (there is quite a good Tesco BBQ sauce, and Asda's French mustard is a good substitute since the EU forced Colmans to discontinue their French mustard), others fall far short (I have yet to find a non-Colmans Tartare sauce which ranks any higher than "terrible").