There have been quite a lot of posts on this lately, it being that time of year so I thought I'd share what I've found out.
You can take Lemsip Cold & Flu which includes a decongestant as it contains phenylephrine and not pseudoephedrine which can reduce supply.
Here's what Kellymom says about phenylephrine and pseudoephedrine
Ds also has a cold and wasn't feeding well so I didn't want to risk lowing my supply further. I was suffering in grumpy, snorty silence until I noticed the ingredients on the Lemsip dh had bought and I hadn't used and checked it out - what a relief!