Assessing the impact of online postal self-sampling for sexually transmitted infections on health inequalities, access to care and clinical outcomes in the UK: protocol for ASSIST,... by Gibbs, J, Howarth, AR, Sheringham, J, Jackson, LJ, Wong, GR, Copas, A, Crundwell, DJ, Mercer, CH, Mohammed, H, Ross, J, Sullivan, AK, Murray, E, Burns, FM