How surgical trainee research collaboratives achieve success: a mixed methods study to develop trainee engagement strategies by Clement, C, Coulman, K, Heywood, N, Pinkney, T, Blazeby, J, Blencowe, NS, Cook, JA, Bulbulia, R, Arenas-Pinto, A, Snowdon, C, Hilton, Z, Magill, L, MacLennan, G, Glasbey, J, Nepogodiev, D, Hardy, V, Lane, JA