IIRC the issue was about the "Battle buses".
The Conservatives claimed them as "central expenses" whereas some local constituencies Election officers seem to be of the opinion that a proportion of the cost should be charged against the expenses of the local candidates.
This would also apply to the other parties, so actions against Conservative candidates would be the tip of the iceberg.
A good example would be if Russell Brand rocks up in a by-election to support a Labour candidate.
Does his hotel and bar bill count against the Labour candidate's election expenses ?? (which is what they spend mainly on posters and leaflets).
This is a grey area that needs clarification at a National level