As far as I'm aware there is nothing in the rules stating that you need to resit an IFR law exam or do a flight test. You just need to get up to scratch to do an IFR renewal flight.
Although obviously it would be wise to read up on the IFR law stuff before this as you will be questioned on it and if its been a while then a lot may have changed.
Have a read of CAA part 61 it's all in there anyway.
Hope this helps.