Phantom debt from "Sigma SPV1 Limited" via HL Solicitors

Hi guys and girls,

Ok, so a couple of weeks ago I received a letter from a company called HL Solicitors who are claiming to be writing to me on behalf of their client "Sigma SPV1 Limited" for a (phantom) debt of £244 and some pennies. Also, what made me very cautious was the fact that they were offering a 50% discount for immediate payment
(in other words, don't ask any questions, shut up and give us your money and we'll go easy on you. Question anything we say and we'll take more money from you). Also, the letter was undated and sent Mailsort (slowest, cheapest method for bulk letters and.junk mail according to a close friend who works for royal mail)
I've never heard of either company, nor do I owe anybody £244.

So I dropped them an email explaining I'd never heard of either of them, I don't believe I owe the debt and if they believe it does exist then I want a copy of the contract I signed or a bill of sale for goods and/or services I have received to the value of the debt. I also mentioned that if I didn't receive any documents by post or email within 7 days, I would consider the matter closed and any further letters would be returned to the sender.
20 minutes later (this was last money afternoon) I received an email back basically saying "thanks for contacting us, I will pass this onto my team who will look into this and be in contact shortly".
A week passed..... Nothing. No email, letter or phone call.
So, I emailed them again to say that the 7 days had passed, I had received no details of the debt by post or email so I consider the matter closed and any letters received from now on will be returned to the sender. Bearing in mind they only took 20 mins to give a basic reply last time.... I received no reply.
I google'd Sigma SPV1 Limited and they have no website, no phone number, no indication of their line of business and is registered to a semi detached council house in Cockermouth.
Could this whole thing be a scam??
How difficult can it be to send out thousands of letters by Mailsort, each offering a 50% reduction on an urgent debt to a company that barely exists? How many people would pay up just for the sake of it (Its only a couple of hundered pounds darling, better than ruining our perfect credit rating....). And even if somebody does speak up and say they don't owe it, they just forget it and move onto somebody else. And I bet if a regulator got onto them they would claim it was a mistake, wrong address etc and carry on!