Just note that a tender is an invitation to treat, not an offer:
Spencer v Harding (1870) LR 5 CP 561
Reveocation of the tender is received too late; acceptance of the offer (i.e when the legal contract forms) happened before revocation as it came into effect when the letter was put into the letterbox and NOT when received:
Adams v Lindsell (1818) 1 B & Ald 681 (the 'Postal Rule')
Note that the person in the council posting the acceptance must be authorised to do it:
Powell v Lee (1908) KBD 99 LT 284
Show that revocation could have been made by any means (Even if it isn't valid, display your knowledge):
Dickenson v Dodds (1896) 2 Ch D 463
Note that as no method for acceptance has been requested (e.g by fax) then posting a letter will indeed count as acceptance:
Domb v Isoz [1980] 2 WLR 565
I'll agree with dzug's point, but I can't immediately remember any case law supporting it. Try Lexis Nexis. Hope this helps!