I'd hang back a bit. Are the oilies leaking, or just becoming waterlogged, but no water getting through to the inside. With some breathable fabrics, the fabric looses the ability for water to hang in drops on the outside, and run off, so the fabric becomes waterlogged, even though they are still totally waterproof. The usual technique is to wash in warm washing machine with soap flakes, and then tumble dry.
The washing removes the salt, the tumble drying is the important bit, and restores the ability of the fabric to shrug off water.
If the suits are leaking, then Gill have a lifetime guarantee for other than fair wear and tear.
If they do need waterproofing, then Gill should be able to advise.
In all three cases, I'd phone Gill customer services on 0115 9460844 and ask for advice
Can't praise Gill enough...I managed to melt the back of the legs of my waterproofs, sent them back to be repaired but asked that while they were at it would they sort out a leak in the crotch area.... no questions asked and I got a brand new pair almost by return of post. Excellent service.