When you are dealing with gifts or loans between family members the absolute rule is to document it.
If your father were to pass away and you claimed that there was an outstanding loan without any documentation then HMRC could well be suspicious of this and minded to ignore the debt in any Inheritance Tax calculations, however if you were able to produce a loan agreement and evidence of the transfer of funds (bank statement) then they will allow the deduction from the estate.
Similarly any repayments your father makes will be treated as such but without the documentation could be treated as potentially exempt transfers from his estate and therefore subject to the 7 year rule.
All of this assumes that his estate is likely to be above the inheritance tax threshold, if not then it is a mute point as no tax would be due in any case.
There is no set amount that a Bank is obliged to report to the authorities, however they are obliged to report any suspicions to the serious and organised crime agency via a Suspicious Activity Report so you should be OK on that score.
If you want more advice or a professionally drawn up loan agreement then my e-mail is in my profile.
Hope that helps
Nick.