Buying a synthetic sleeping bag would be a waste of money in my opinion (heavier, bulkier, less comfortable, less durable, unnecessary). As Pete says, dew is not a big deal. The only time when getting a sleeping bag damp is a problem is when doing Alpine-style serious routes over a number of days.
There are a few things to think about:
1) Where has the moisture come from? The Xero 550 is a pretty warm bag for summer use, so it may well be from you sweating into the bag. If its coming from you then you need to sweat less - ie. wear less clothes, open the zip, etc.. Moisture is more likely to get stuck in a bag if there's high relative humidity in the tent, so keep the tent ventilated. Tents with mesh inners are a good idea, and for UK-use a two-skin tent is a must.
2) If there's very high humidity outside the tent, and it's warm, then getting moisture out of the tent is going to be difficult as there will be little in the way of vapour pressure gradient. In this case, venting will make no difference.
3) How wet is the bag getting? If it's moisture on the outside of the bag there's no problem at all. If it's getting into the down then this is not a disaster: down dries out. Down is way more durable than most people think, and it does not turn to 'porridge' unless it is completely soaked (ie. drenched), and then compressed while it is soaked. Down is amazing stuff - despite its greater density, it floats on water and to get it really wet requires quite a concerted effort. If you're worried about damaging the bag or the filling, don't be unless it is really really wet. Even then, you'll have to crush the down super-tight to make any lasting impression on it. As far as thermal properties go, a damp Xero 550 will still massively outperform any synthetic bag on a warmth-weight ratio.
4) A synthetic bag of the same warmth as a down one will not absorb as much moisture, but as a result may well feel a lot sweatier. When wet, both synthetic and down lose some warmth.
What's the spread of light like on the Manta? I borrowed a mate's Gamma this weekend but found the spread a bit lacking - it was a quite narrow spot compared to the Petzl Tikka Something-or-Other I had until I lost it... Thanks
How old are you? I ask because you could get yourself on a Conville course if you are a student. They are the best value for money alpine courses available.
If you've been to the Alps before, have Scottish winter experience, and can climb then you're probably already a step ahead of the typical Brit on their first Alpine season! If you can, get a friend who's been before to take you out there, but failing that, getting a guidebook and one of the many alpine skills books is a good way to go. You might have a few 'adventures' that way, but it's the way people have learnt for a very long time. Getting a guide for a day would also be very good, but is obviously more expensive.
There are a lot of relatively easy routes in the Alps, but also some monsters, so start easy.
The trousers are absolutely brand new as they were an unwanted gift and arguably better suited to skiing and winter walking than winter climbing, which is what I wanted them for. The hat's been used a bit but has been replaced by a hat that better fits under a helmet. Happy bidding!