Falsely swearing an Affidavit is punishable under the Perjury Act 1911. If you knowingly make a false statement in an Affidavit, you may be found to be in contempt of court. This is punishable by a fine, imprisonment or both.
As a result, it is crucial that the content of your Affidavit is genuine and truthful. However, as Affidavits are meant to reflect the personal knowledge of the person making the document, you cannot be penalised for:
not including any information of which you were not aware
including information which you believed to be true but turned out to be incorrect
Further, your Affidavit can include personal opinion rather than fact so long as it clearly stated that this is your opinion and not a fact.