Grate glow fire wont light?

I have a grateglow gas fire and had the thermocouple replaced about 2 months ago. Now, when lighting it (with a match as igniter busted) the gas only seems to allow a very small blue flame and therefore the little cap thing on top which distributes the flame 3 ways doesn't have enough coming out of it to stay lit when I let go of the button underneath. Its almost like the gas isn't strong enough to make a big enough flame. If you can offer any advice it would be greatly appreciated. I know if its not an easy fix I will have to get a gas safe engineer out but were freezing and its 8.30pm on a Saturday night.
Thank you x