The obvious reason is some promotional discount or offer expiring.
Check your most recent bills & downloadable contract, this will show all your promotional offers & discounts, together with their expiry dates. The same section will show your standard package price. A £25.50 discount dropping off is entirely feasible.
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Welcome to our Community Forum! Thank you for your first post, and I'm sorry to hear that you're having some billing issues!
You can look into your online billing via your online account, and look at our Bills Explained page here.
As @japitts advised, if your bills have increased, this is most likely due to a contracted discount running out. You would have received an end of contract notification 1 month before your contract was due to finish, along with your contracted discunts.
Please let us know if there's anything else we can do to help.