A- with more random boulders....I've run out of decent ones, and calculations mean I need about 10 tonnes to make the 10m circumference. This would by far look the best, be permanent and maintenance free, but would require a long "shawshank redemption" approach in gathering suitable boulders and bringing them back week/year in year out. Local quarry stone doesn't match this wall, most of which was hand excavated and the best stones kept aside. B- Dense concrete blocks- £450 should see it done, could keep the semi circle nice and accurate, render it, add some feature holes for plants/lights, taper the boulder wall into it so they look like they merge and a likewise start a small one where the other side ends. This is the cheapest option, but could either end up looking very good or total dogs dinner... C- Sleepers concreted in at equal distances around(9 of), with 1.2m battens to bridge between each. Would look ok but not a continuous curve like the other two, requires digging 9 2ft deep holes in bouldery gravel glacial till, and accurate setting out of each sleeper or could look gash. Potential for rot and replacement problems. Might just look like shuttering. My heart tells me A is the answer but I'm an impatient sod!!! Any other ideas folks? fire away. The wall as it is took a day to build and the material wasn't graded, it's pretty rounded stuff, not the usual nice split plane stuff you usually see in drystone walls, but I like the idea it came out the ground and was built there...!