Yellow caterpillars in old bees nest

[Bill Grange]

I have just had a number of yellow caterpillars brought by an enquirer
into the museum where I work. They were inhabiting a plant-pot which was
previously colonised by bumble bees.  The caterpillars are about 1.8 cm
in length, with well developed 'true legs' and four pairs of 'false-
legs' or 'pro-legs' (so I assume they are moth, rather than sawfly). The
segments are very distinct. The body coloration is a strong yellow, with
a brown head - no other obvious markings. The most remarkable thing is
that they have constructed a silken mass of extremely tough silk,
consisting parallel-running cylindrical cells.  

I would be extremely grateful if anyone could offer an identification.

I am in Great Britain
-- 
Bill Grange