Hi Philippa, definitely sounds like it could be a failed reed to me. This does happen from time to time, and when it does, it’s best to replace a whole reed plate.
For the UK, you can order reed plates from a company called Sontec. Go online and order here:
http://www.sontec.co.uk/on-line-services/order-parts-accessories/
They’ll get back to you with a quote. I recently ordered 8 reed plates;
3 x W4110210 High section reed plate
3 x W4110310 Middle section reed plate
2 x W4110110 Low section reed plate
The total, including VAT and postage was £122.81
They came from Japan, and took a few weeks if I remember correctly.