Before you ask that question, you should consider some things - 1st - Signal quality is more important than signal strength so how is your quality looking?
Do you have any picture/sound problems at the moment? If so, then you may have an installation issue. If not, then maybe best to leave well alone.
If you have reception issues then here are some things to think about before looking at a new LNB. You can get your dish realigned (it may have moved since original installation). Depending on where you are in the UK, you may find that getting a bigger dish makes more of a difference than simply replacing the LNB.
How long is the cable run from dish to receiver? The longer it is, the more important it is to have a good quality cable.
Also, the signal strength and quality bars on the Sky Box are not necessarily the most accurate in the world - best to have a proper signal meter take a reading from at the dish to confirm correct installation, then use the same meter at the point the cable arrives at your box to see how much degradation there is (if any).
But like I say, if your TV viewing is currently unaffected, I would advise you leave everything as it is and don't tweak for tweaking's sake!