Thanks for all your advice. Fortunately the lump seems to be going down now.
Staffyluv, I had discussed a needle biopsy with the vet and we'll be getting one done if the lump hasn't disappeared by the time we go back.
Dan, thanks for all the info. I'll definitely have a look online for some more information.
InspectorRex, the web address above doen't seem to be correct. And I can't find the correct one or anything about reporting vaccination reactions through google.
Thanks again everyone who responded