Mole grips will be best, its very likely to round if you try and start it off with a normal spanner.... I found that out the hard way....many times! 
Do you have a Halfords trade card? I think the brake pipe spanners from there were reasonably priced on trade. Its also a funny size I think, 9mm maybe? Cant remember now.
Do you have a Halfords trade card? I think the brake pipe spanners from there were reasonably priced on trade. Its also a funny size I think, 9mm maybe? Cant remember now.