Quote: Marc P @ April 2 2009, 8:48 AM BSTRandoms sitcom has lots of main mail characters.
I like what you did there, by changing the 'letters'. 
One other thing about this script, when the character is speaking on the phone its usually best to put (on phone) in brackets just before the dialouge. Although it is obvious later on when she is on the phone, that very first bit may confuse someone who isn't paying full attention. 
Found the opening scene confusing - I had to read Kate's dialogue three times to make sense of it: too much information, in no particular order, and interspersed with weak unnecessary jokes. Try re-ordering and editing it down. e.g.
Do you know what time it is?... Is it? O. Right. Sorry.... Your what? Don't you check them regularly? Nick used to check his all the time.... But it's only twisted right? It's not come off or anything? Matt, you're a chef not a gigolo. What am I supposed to do about the wedding?... A fixation? How long does that take? O, they're doing that to you... right now... Ergh. Yep. No. Call me later....
In the second scene Jonathon is just annoying, if you are going to have a character who is relentlessly facetious he has to be actually funny. It is also a bit flabby. Take this bit of dialogue:
JONATHON
So, what's your problem?
KATE
Massive cock up.....
JONATHON
Finally. Well done. [WEAK GAG]
SHE IGNORES HIM.
KATE
To cut a long story short....
JONATHON
First time for everything.... [WEAK GAG]
KATE
Look, this is serious.... [IS THIS NECESSARY] Matt's called in sick. Twisted testicle.
JONATHON
I'm not surprised if you speak to him like that. [GOOD]
KATE
He has... a twisted testicle. [UNNECESSARY]
JONATHON FACE LOOKS PAINED.
JONATHON
How'd he get that?
KATE
I don't know. But I intend to find out. And when I do, I'm going to twist the other one for him [STOP NOW, END ON THE GOOD GAG]for landing me in the.... Mire.
JONATHON
Can they do anything?
KATE
Still checking. But he'll probably [ DO YOU NEED 'STILL CHECKING' OR 'PROBABLY'?] need a testicular fixation [STOP NOW, THE 'OR SOMETHING' JUST CONFUSES] or something.
JONATHON
Haven't you got one of those? [SOMEHOW THIS GAG DOES NOT FEEL REAL - GIRLS MIGHT GET FIXATED ON COCKS, BUT NOT TESTICLES. MEN HOWEVER DO TAKE AN INTEREST, IN THEIR OWN AT ANY RATE, SO MAYBE THAT IS A BETTER ANGLE ON THE GAG.]
KATE
This is no time for jokes. [NEVER EVER USE THAT LINE IN A SITCOM!]
JONATHON
Who's joking? [OR THAT]
On plus side Kate is an appealing character, and there is something to work with here if you lose the weak banter and tighten it up.
Thanks Timbo. That was very useful.