Transmission spectroscopy is a powerful tool to probe exoplanetary atmospheres. Here, we study HAT-P-11b, a warm Neptune (~800 K) planet on an eccentric orbit that has a significant water feature in the HST WFC3 bandpass. We re-analyse previously published HST WFC3 data and Spitzer data along with new data from HST STIS using a global consistent framework 'ExoTEP'. The STIS data is useful for characterising the scattering properties of the atmosphere. This work has been accepted for publication in The Astronomical Journal (AJ) and a preprint is available at arXiv:1910.07523.