The rules allow you to operate within 25m vertically of the summit. Great Orme is 207m ASL, so this implies that you may conduct your SOTA activation anywhere in the area above 182m.
To be safe, you could make that 190m, then it is straightforward to look at the 190m contour on a map and operate from within that area.
Great Orme is a horrid location for QRM on VHF. The following are some of the techniques used by activators to counter the QRM:
- Move away from the summit within the 25m activation zone (as indeed you suggested)
- Use a beam antenna
- Use horizontal polarisation (even if just turning your handheld on its side)
- Use 2m SSB
- Use HF
- Use an FT-817 (or similar) - a decent rig will withstand the mush better than a hand-portable
I can personally vouch for all these techniques. As I say, it is an awful location for VHF, and most of these won’t eliminate the problem - but they will help you cope with it better and allow you to get your 4 (or more) contacts.
For further info, check out the links on the summit page at http://sotawatch.org/summits.php?summit=GW/NW-070