The previous weekend I cleaned the arches really well and lagged them in Hammerite underseal with extra waxoyl (picked up two years ago for £1.89 per Schutz litre - I emptied the shop at that price!).
The rears went in nicely. Thanks to KINGPRAWN on here for the tip about riveting them in place (see pic below).
The fronts were a struggle but went in the end. I went for a drive afterwards to check that they weren't flapping about at any speed. That was OK but when I got back and turned on full left lock the front offside fouled on the tyre rim. I will probably cable tie that out of the way.
I will be keeping an eye on whether any wet mud gets caught behind them - which would defeat the purpose. The fronts look like they may be more susceptible to this.
Pics:
Rear arch with underseal.

Liner fitted with rivet to hold it in place.

Red is reflection of my overalls!

I'd be interested to know what other's experience is of them.
Pete