Two new measurements of diffractive DIS events (ep \rightarrow eXp) where there is a large gap in rapidity between the hadronic system X and the leading proton p are presented, measured with the H1 detector at HERA. The first technique measures the full final state including the leading proton. The t dependence of the data, parameterised by d\sigma /dt \propto e^Bt gives a value of B = 5.0 \pm 0.3 (stat) \pm 0.8 (syst) GeV^-2. The second, higher statistics, measurement technique selects diffractive events on the basis of a large rapidity gap separating X and p. The reduced cross section \sigma _r^D(3) (=F₂^D(3) if F_L^D(3) = 0) is extracted and good agreement is found between the two techniques. A value of \alpha_P (0) = 1.173 \pm 0.018 (stat) \pm 0.017(syst)_-0.035^+0.063(model) is measured for the effective pomeron intercept and the Q² and \beta dependence of the data are described by a NLO QCD fit to the data including a Regge factorisation hypothesis. The logarithmic Q² derivative of the ratio of the diffractive to the inclusive cross section is measured and found to be consistent with zero in the bulk of phase space.

PACS numbers: 12.38.Qk, 13.60.Hb