Minty....got it now.
The Ultrasound will confirm if it is a fluid filled, which will confirm the diagnosis of Baker's Cyst or solid, which will mean back to the drawing board.
Assuming the diagnosis is correct then it will mean a needle inserted in to the cyst and fluid drawn out and then probably Cortisone injected into the space through the very same needle.
Some doctors anaesthetise the skin before inserting the needle, others don't bother as it is so quickly inserted.....just like an ordinary injection.
Never had it done myself, but it is regarded as "non painful"
Good luck.