Problem with part of a Lighting Circuit

Hi everyone,

I've been scrambling around my loft today trying to resolve a lighting issue.
The Master Bedroom light dimmer light switch was spongy so I replaced it today.

Following replacement, however, the ensuite extract fan and lights won't work, neither will a ceiling light in another bedroom. All other upstair lights are AOK.

I'm struggling to work it out and can't see how replacing the dimmer switch (which I've done before with no ill effect) has resulted in this issue.

Any ideas?

I've checked fuse box, all appears AOK.

The ensuite lights are supplied via a Kaoyi Transformer 220-240V primary, 11.4V secondary. I tested the supply cable to this Tx and it looks like the bathroom light switch itself may be the cause as I don't appear to be getting 240V when I switch on the bathroom light at the primary side of the Tx.

My next plan is to open the switch and see if it's a fused switch, although there's nothing external to suggest that it is, just a normal two gang switch.

Thanks for any advice