I have recently purchased a set of bullhorns that have a 19mm ID. My Cane Creek 200TT lever will not fit in the end, does anyone know of a brake lever that will fit (preferably one with a return spring, unlike the dia compe 188) ? The Tektro 4.1 specify they will fit 19mm ID handlebars, but does that allow any space for tucking in the bar tape ?

You might want to check that you've taken off the spacer that helps it fit into larger-diameter bars. I had the same issue with my Tektro 4.1s but I hadn't realised that there was a spacer on there.

I checked, there is no spacer, the expander plug is just too big to go in.

I could not fit the tektro 4.1's into my handlebar except by completely removing the expander plug and twisting it in. I'm not sure what the diameter of the ends is, but regular plugs go in just fine. I didn't like them anyway and just ended up using a cross-lever.

I guess it's going to be the age old persuasion technique of a file and a hammer!