unknown plant

Observation - unknown plant. Description: No photographs - the plant was growing in deep shade in woodland, and (combined with an overcast sky) it was too dark for photography. Small (maybe 15 cm high) decumbent (possibly ascending - I didn't note this at the time) plant with opposite leaves a

No photographs - the plant was growing in deep shade in woodland, and (combined with an overcast sky) it was too dark for photography.

Small (maybe 15 cm high) decumbent (possibly ascending - I didn't note this at the time) plant with opposite leaves and small yellow flowers. No obvious indumentum on any part of the plant.

Flowers solitary in leaf axils on long slender pedicels. Corolla pentamerous, choripetalous (or nearly so), about 8mm across. Calyx of 5 free linear sepals. Stamens estimated at 10. Thought to be one carpel, but I couldn't get a clear view.

Leaves opposite, elliptic, with an appearance that could pass for Epilobium or Lysimachia.

I've been through the illustrations in couple of books and didn't find any plausible candidates - the nearest was Saxifraga, but it's not a match for any species.