The ideal thing would be to make a starter with it, although youre wanting to brew tomorrow. That would give you a decent idea on the viability of the yeast and boost the cell count which might be needed after the stress of the possible heat exposure. Chances are its still good, however that is dependent on how long and how hot the exposure to the heat was. When I used to brew with kits I ordered them with yeast as well until I brewed one once and got dead yeast. No fault of the brew shop I ordered from, I just live in the heat of FL and that can be rough on liquid yeast. I now purchase my yeast from a local shop or use dry yeast. The bottom line is you won't know until after you pitch it. It may take off like a rocket, it may have a slow sluggish start due to the health of your yeast, or it could not do anything. If you have a local shop you could get more or order dry online, although I'm not sure of any equivalent for Burton. Give it about 72 hours and if the gravity doesn't drop, I'd repitch. I hope that helps somewhat and best of luck.