I am presuming 2 things here with the postal rule,
that the trail t&c stated that cancellation had to be by post, if this is the case there is a clear read across for the postal rule, there is recent case law on this subject (2005/6 but I really would need to look it up, and TBH I dont have the time to at work and do not access to resources at home).
And that the contract did not state something on the lines of cancellation must be in writing to this office and only accepted on safe delivery to this office
What you are looking for is something like this All cancellations must be in writing to this office
If that is the case in the first instance you could write something like the below:
Dear Sir/madam,
In reference to your letter dated xx, I deny all liability for this debt.
I have already explained xxxxxxxxxx.
I have made contact with Mr XX {the ex employee} and he has confirmed the above and stated he is willing to make a witness statement to court if need be. (only put that in if true)
Whats more I refer you to the postal rule and your clients stated that a cannacellation has to by writing in the post in their terms and conditions. Byrne v Van Tienhoven (1880) is clear that if the letter were to be lost, acceptance (in this case rejection) has still taken place. I must stress that it was open for your clients to specify how the trail was to be cancelled. English case law is clear that your clients in choosing the postal system they also accepted the risks inherent with the postal system and as such should also bear the risk that the letter may get lost or delayed.
Furthermore I will more say if you pursue this matter in the courts I will defend the matter in full and I suggest that legal advice is taken before they proceed further.
I await your position.
That may be just the trick to get them off your back, but if it is not I could only suggest that you go get legal advice as it is more than an informal forum can deal with.
Or my all time best that I love
Dear Sir/Madam,
We refer you to the reply given in the case of Arkell v Pressdram
Google it. Arkell v Pressdram
Also if it was by fax, email or post it may just not have reached the relevant members of staff this would be an internal failing and could not affect the validity of service.
If that is the case the letter would need to be clear that you see the matter as an internal failing on their part and that you cannot be held liable for this.
There is plenty of ways to skin a cat but you would have to post here the trial offer t&cs and a copy of the rejection.
Without that it is all really moot, and remember that I am not insured to advise you and take no liability for my advice in an informal forum and any questions should be addressed to an insured legal professional
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I fact I just type due the fact telly is rubbish.