Will a Shimano 10sp tiagra front triple sti unit work on an ultegra 10sp double chainset? Will i need a different mech?

Also, will an 11sp chain run on Ultegra 10sp chain rings?

Thanks

Yes.
No, just set the stops correctly.
Probably, might wear a little quicker than it otherwise would.

Ace, thank you!

As above just set the stops on the mech correctly and use the first two positions on the sti – you’ll never get to the third click.

A few years ago, 105 5600 era, quite a few bikes came set like this as stock – triple shifter with double cranks.

On the winter bike I’ve got a 105 triple shifter on a Sora triple front mech with 105 double chainste, it’s all fine, just a ‘dead shift’, ie last click of the lever it moves but nothing happens
On the posher bike I’ve got a Tiagra triple shifter with a DA double front mech and double DA cranks, same things, it’s all fine apart from the dead shift.
No idea about the chain, but a 10sp one seems fine on 9sp rings if that helps

yes you just have a dead click as the front mech controls how far it can travel

Thanks all…thats great