The CHESS trial: protocol for the process evaluation of a randomised trial of an education and self-management intervention for people with chronic headache by Nichols, VP, Ellard, DR, Griffiths, FE, Underwood, M, Taylor, SJC, Patel, S, Bright, M, Carnes, D, Davies, B, Eldridge, S, Ellard, D, Evans, S, Griffiths, F, Haywood, K, Hee, SW, Matharu, M, Petrou, S, Pincus, T, Sandhu, H, Taylor, S, Wilkie, A, Higgins, H, Nichols, V, Patel, S, Potter, R, Boss, D, White, K, Achana, F, Mistry, H, Caldwell, F, Probyn, K, Team, CHESS