Yes layer can be laid on layer if mineral wool.

A very real danger applies if the build-up exceeds the depth of the ceiling joists. If a surplus of mineral wool is above the joist height over a large area and covered by boards, it can and will push the ceiling down!

Perhaps worth thinking about?.

I believe the government subsidy scheme is due to end this month or very shortly. If you or someone else in the home 'fits the criteria', then you can get this done for free.

Be warned though about the approved company you might select, some are very fussy about anything in the attic, which may hinder their speed and bonus!.

On a practical note, new insulation should be laid at right angles to the existing insulation to avoid any matching gaps where the roll widths meet.

This poses a problem in that a new roll, due to it's diameter, cannot be unrolled under the lower part of the A trusses and if rolled out first in a place nearer the apex, will not then slide over the top of existing insulation to get it into it's final position.

I eventually gave up on the cross laying technique and followed the same line as the original. The rolls were cut down to match the width between the trusses and layed tightly together avoiding any gaps. The process was then much easier.

A solution to the cross laying problem would have been to lay a film of polythene first for the insulation to slide on, but I didn't have any sufficiently sized.

Don't under-estimate the effort involved. It's hot dirty work, made worse by wearing a mask and goggles and the rolls are heavy and ours were a tight fit through the loft hatch, having first man-handled them through the house and upstairs from the garage where they were stored after delivery.

'pointing pit' -)

When we purchased the house we live in some thirty years ago, it was only 12 months old. On inspecting the loft, I was surprised to find black mould developing on the roof timbers.

The reason for this was a lack of ventilation, the moisture content in a new house and a central heating system that vented to an expansion tank in the loft.

The builders were called back and they replaced one of the ridge tiles with a ventilation type and cut a hole in the roofing felt. This cured the problem, the mould disappearing within a few months and no further problems in the following thirty years.