Hi Sophie,
thank you so much for detail pics.
The shape, the 4+1 bubbles and the used fillings are consistent with Paul Ysart's Caithness time&style and material, but- confusion does not get better:
there seems to be a throughout fill of translucent blue layer in the whole area above the bottom, and this is absolutely
unusual - not seen so far in Paul's harlequins;
this type of filling is common and usual in the later (mostly CG or PH signed or unsigned, and acid bottom etching CAITHNESS HARLEQUIN in variations )
harlequins at Caithness, ca. 1974 and later, after Paul had left! Maybe it is an early frigger of Caithness, using up remaining PY colour & cane material?
best wishes, Erhard