The High-Frequency Instrument (HFI) for the Planck Surveyor mission will measure anisotropies of the Cosmic Microwave Background (CMB) down to scales of 6 arcmin and to an accuracy of /∆T/T=2×10-6. Channels ranging in frequency from 100 to 857GHz will use 100mK spider web bolometer detectors with NTD Ge thermistors. The detectors must be photon noise limited and fast enough to preserve signal information at the 1r.p.m. scan rate of the satellite. The prime low-frequency CMB channels at 143 and 217GHz are the most technically demanding owing to the lower background limited NEPs. For the 143GHz channel the requirements are that the time constant /τ<5.7 ms and the NEPbol <1.53×10-17 WHz-1/2 including contribution from amplifier noise. We present here thermal, electrical and optical data on a prototype detector which, although optimised for the 100GHz channel, satisfies most of the requirements of the more demanding 143GHz channel. The measurements are consistent with ideal thermal behaviour of the detector over the appropriate bias and temperature ranges for optimum performance. From optically blanked electrical measurements we determined the dependence of resistance and thermal conductance on temperature over a wide range, 70-200mK. The optical responsivity and NEP were measured under photon background conditions similar to those expected in flight. Measurements of speed of response as a function of bias at different temperatures allowed us to determine the variation of total heat capacity with temperature. Extrapolation of these data show that in principal performance for all the Planck HFI channels can be met.