have you tried putting those water-retaining granules in and under the soil for the 2 more exposed to the sun?

or some subtle shade ofcourse.

not tried the granules but I do give them plenty of water. think it may be because of the length they have grown to as to why they drooping.

Are the tips of the stems turning over like an inverted"U", Nick? Or is each whole stem tipping over, and maybe turning up at the tip.

If the former, it is likely a watering problem, especially if they perk up when watered. If they don't perk up when watered, it may be a problem with root rot, which often attacks plants in a seemingly random fashion.

If the latter, the stems may have gotten too long to support their own weight, though that usually happens more often in the shade. Another possible cause is if a watering was forgotten, the stems might have drooped enough to take that position permanently, and that is more likely to happen in the sun. You may get a lot of side stems growing up from the prostrate ones, which will make it easier to prune back into shape, later.

I'll try giving them some more water then or put some pebbles on the soil to stop it drying out so fast.

i hope the wall has a lining in it.the water will be taken in by the bricks
slightly if they are in full sun
dont forget lavenders do go a bit woody after awhile i have quite afew,but i like the look of yours,i might have ago at that.