When the Save Bedford Hospital party registered with the Electoral Commission, it was made absolutely clear to us what the rules are, and what the penalties are for not following them. I (as party treasurer) get a letter every three months reminding me to update our declaration on both donations and loans, and another reminder if we don't respond to the first one.
The Electoral Commission has a web site which explains all the rules very clearly. The staff, if you phone them, cannot be more helpful.
In summary, it is not possible to get things wrong "by mistake".
Our party is run by a small bunch of amateurs, and we can get things right without too much bother. If they can't run the Labour party, can they actually be trusted to run the country?