deadline for ESA 50 return Royal mail tracking

Hi just a warning to anyone who is sending in any correspondance to DWP or any other benefit departments for that matter. I sent off a completed ESA 50 along with a bundle of suporting evidence via 1st class recorded signed for next day delivery on Monday and its now thursday and still Royal mail system showing error in system . I spoke to royal mail after waiting 25 minutes on hold to be told that the system will not show this item until it has been signed for, and it could take up to 10 DAYS !! I have never heard anything so ridiculous , I re checked on Ryal mail site and it specifically states we aim for next day delivery including saturdays.
I posted it from my companies mail room so the item has been franked and I have receipt of postage, however what royal mail are not fessing up to is that they are having a huge problem with their new revamped tracking system since introducing the captcha format = ticking photo boxes of items prior to using search facility. they are blaming "adverse weather conditions" which is only partly true. I have contacted DWP andthey said this claim will not be affected , however I do not believe a word they say. I am looking like missing the deadline despite me making every effort to return forms by recorded 1st class.
Anyone else thinking of using royal mail may want to think again at the present moment as my letter is lost in the ether somewhere and I may be penalised for this albeit due to circumstances completely beyond our control. Just a warning for others. Best wishes to all.