C,
No black and white answer here.
Depends on what is going on and how bad this problem is can make the difference between no problem and a no-no (although would have to be pretty bad for the latter to be the case)
A lot of people are susceptible to the occasional bout of sinusitis and get by well the rest of the time and just have to lay off the flying for the week or so that it takes to get over this.
Some people however suffer with chronic sinusitis which commonly is caused by some allergy or chronic irritation and can be accompanied by nasal polyps.
The staple treatment for this is topical (local) steroids but sometimes people will need some surgery as well especially if the polyps have become big(gish).
So it really depends on how badly you are affected and what the cause is and how effective treatment will be.
If you have very bad chronic sinusitis chances are that you will not enjoy flying due to the discomfort you will get whilst ascending and more so descending.
Talk to your ENT surgeon about likely successrate of planned treatment and once that is established talk to the CAA @ Gatwick.
Best of luck
FD