Take the garden hose to it. Shoot those critters off with a powerful jet spray from the garden hose.

Whatever you do, don't take it outside and jet wash it....that'll cause more damage than the mealy bug.

Personally, i'd persevere with the meths if that's working.....if you do use any spray (Provado do a decent insecticide) you might want to test a small area first to see if it scorches.

Maybe do the same with a dilute fairy liquid spray....mealy bug are similar to aphids as they have a waxy coating. Soap destroys that and kills them.

The garden hose works fine for me and no caustic chemicals or scorching. Just a good water stream strong enough to wash them off.

A really difficult one to get rid of. I would try Provado Ultimate Bug Killer2 - aerosol; it has a different formulation from the spray as it deals with red spider mite and reckons to sort out mealy bug.

I've checked a number of reliable sources and this is what I found:

The Water Cure

One of the first attempts you can make to kill mealy bugs, and one that works well in the early stages of infestation, is to simply blast the bugs off with a strong stream of water. To be effective this treatment should be repeated on a daily basis until the mealy bugs are gone.

Dishwashing Liquid

For larger infestations, try spraying with a mixture of dishwashing liquid and water. Use equal parts of each and stir to mix rather than shaking to avoid excess foam. Spray all infected areas. The soap coats the mealy bugs and effectively suffocates them. It also breaks down their protective waxy layer.

You can rub the leaves with a soft cloth after spraying to remove the bugs, or leave the solution overnight and then attack the weakened bugs with a strong jet of water.

Rubbing Alcohol

Spot treat areas of mealy bug infestation with a cotton ball soaked in rubbing alcohol – simply dab the critters and rub them away.
Insecticidal Soap

Made from a mix of mild detergent and pyrethroids, insecticidal soaps are applied to the plant, its pot, and the immediate surrounding area. Insecticidal soaps are generally safe to use on greenhouse vegetables (but always check the label).

You can also mix insecticidal soap and horticultural oil together as an alternative approach to killing mealy bugs – use 1 teaspoon soap, ½ teaspoon horticultural oil and 1 litre of water and apply with a spray bottle.

Try avoid using caustic chemicals - they are dangerous to have around the house - pets, children. It washes down into rivers, lakes and kills the fish.

Sorry for the late thanks - But many thanks folks for all your input - Now that the weathers turned a bit warmer I've been able to pop the Fern outside - so I've been trying the garden hose method for the past few weeks! Pleased to say the mealy bug infestation has reduced and the fern is looking much happier! So I'll keep up the hose treatment for a good while just in-case the little blighters are still lurking?
Cheers