TBF telling you the law could have been helz way of showing that they actually care, i mean if helz didnt and you tried to help your cat and caused it a little bit of harm and you then took the cat back to the vets and one uppity veterninary nurse thought you where abusing your cat (im not saying you have/will/or are inhumane enough to do that) they have a right to report it.

TBH a lump could be anything though. My girlfriend often says to me "Iv'e got a lump" and i think "OH S33T BREAST CANCER!" and it turns out to be a heat lump or something.

BUt again the best advice is take your cat the vets elderly cats need more care than what they used to (obviously) and being that you and (im guessing) not many people in here are actually trained vets then its better to be safer than sorry and take him in.