Possible rare warbler

A member of HOC reports: We heard a very odd mechanical-sounding song unlike anything I've heard before coming from a bird hidden in thick undergrowth - umbellifers etc - beside Riddings Brook. We were on the public footpath from Knill just before Lower Harpton Farm GR SO 27848 60329. Slower, lower and more staccato than Grasshopper Warbler (with which I am familiar) and without the "head turning" variation and terminating in a very brief "sedge warbler like" burst with more varied notes (unless two birds present of which one WAS a sedge warbler!). Mechanical trill repeated a few times. Possible glimpse small "brown job" in flight along ditch. Could just be an odd sounding sedge warbler as habitat suitable for this species but I have never heard a sedge warbler make such a long, mechanical-sounding, constant in pitch and tempo, song component. Listening on Xeno-Canto River Warbler sounds right - but (as well as being v. unlikely due rarity and habitat) that doesn't account for the the "sedge-like" notes following the prolonged mechanical trill.

MCC wonders if Savi's Warbler might fit the bill.