Helium atoms travelling in pulsed supersonic beams have been photoexcited from the metastable 1s2s 3 S 1 level to Rydberg states with principal quantum numbers exceeding 400 by resonanceenhanced two-colour two-photon excitation using narrow-bandwidth CW laser radiation. To achieve this, the photoexcitation region was shielded and stray electric fields cancelled using a cubic arrangement of 80mm×80mm copper plates. The excited Rydberg atoms were detected by pulsed electric field ionisation downstream from this photoexcitation region. Comparison of the experimental spectra with the results of calculations indicate that the stray electric fields at the position of Rydberg state photoexcitation were reduced to as low as 160μV cm -1 . Using helium in these experiments minimises temporal changes in the stray fields resulting from surface adsorption. Measurements performed 70 h apart indicate that the rate of change of these fields was on the order of 2μV cm -1 per h, making beams of metastable helium atoms ideally suited as probes for high precision electrometry in spectroscopy experiments involving less readily available species such as positronium or antihydrogen.