Whilst I agree with Cem that the weather is cack, and the job is not THAT urgent... prolonging it for a long long time could cause the exhaust gasses to keep cutting somewhat of a channel in the ali head (I've seen it pit them a little before).
I'd be happpy to do the job, but only really if the car could be left with me. I may be the exception to the rule, but my method of attack is just to whip the cylinder head off, and change the manifold with the head out of the car.
Yes it's quite a bit more work, but MUCH more preferable to struggling and rolling around on the driveway.... and makes it much easier to porperly flatten the surfaces etc to prevent it happening again, also makes it easier to clean the threads out, and tackle any stubborn nuts...
James, add me to your exception..
working under the car in an evil tight area , makes the job unnecessarily long and hard.. and whilst the heads are out do everything once , pay once ..

Personally I think it would be asking the customer to pay twice possibly three times more to make it easier on the person doing the job 
Are you implying I would rip someone off?
I woudn't "ask" anyone to pay more than the job was worth. And if I did it in a way that made it easier for me, why are you assuming I would be passing that cost on to a customer? 
No, i'm just saying why go to the expense of replacing a cambelt kit, water pump, camcover gasket and O rings, and a head gasket just to do an exhaust manifold gasket. I would charge around £200 to do a passenger side manifold gasket and if you can remove a head and supply and fit all those parts for lets say £300, I must be doing something wrong.
We could go around in circles on this one forever.
You could argue if the cambelt kit is near due, and camcover gaskets are past their best, the owner would make a huge saving on labour having it done in one hit.
I wouldn't fork out myself for a cambelt kit (unless the owner wanted to provide one), but I WOULD provide a head and camcover gasket, if I used the head removal method to make it easier for me.
So -
Manifold G - £10
Head G - £10
Cam Cover G - £22
Misc (doughty washers, bit of sealant etc) £10
Parts cost £50.
So if charging £200, you'd still make £150.
I'd rather make £150 leisurely doing jobs I enjoy, up top, than rolling around on the floor, fighting in a tight space.
It's each to their own really mate. There's nothing stopping people saying "no thanks"