Hey Jules, In case you do want to go for it yourself. I would just get some proper diamond bits for porcelain tiles. Something like this - https://www.screwfix.com/p/erbauer-diamond-tile-drill-bit-6-x-67mm/51908?tc=NA8&ds_kid=92700055262507123&ds_rl=1244066&gclid=EAIaIQobChMIgtaHnYOD7QIVQeztCh18fQSuEAQYBSABEgKlp_D_BwE&gclsrc=aw.ds A regular impact driver drill should work. I must warn you though, it’s a slow process drilling each hole and you need to make sure the bit stays cool (have a spray bottle handy or lubricant oil). Good luck!

Oh gosh Anita that sounds like a nightmare. We bought a new set of drill bits, these from Screwfix: https://www.screwfix.com/p/dewalt-straight-shank-masonry-drill-bit-set-8-pieces/3189x We were using the number 6 or 7 I think so you could also look for a single bit in the correct set - at the time we weren't sure so bought the set but I doubt the larger sizes will get used so we wouldn't do that again. We did also buy a set for metal when we originally thought it was metal but after drilling a couple we are pretty confident it's concrete now. We had the same experience that you drilled in by around 1.5-2cm, basically through the plaster and then you just stopped completely, my partner just his kept at it using quite force (which is where the the 2 handed drill comes in handy for support) and eventually we got through. If you are raising your curtains could you try even higher or are you going right up to the ceiling? Or could you go slightly wider? We were setting our curtain pole brackets around 10cm from the edge with the curtain pole over hanging a further 5-10cm depending on the window. We found on our windows that we'd occasionally get 1 hole that would drill really easily, we think that must have been hitting just to the outer side of the concrete lintel. Although to be fair in your situation I'd maybe be tempted to go with the batten now, it was a pain for us to get this to work and at points we were wondering if was worth the effort. Once the curtains are on you can barely see any of the fixtures anyway so it might be worth considering or maybe get some professional advice if you can? Like you said it's not worth buying more tools for, a handy person with the right tools would get the job done in easily less than a hour.

I had to fit shower & shower screen last week, purchased a diamond bit from Toolstation, worked very well. https://www.toolstation.com/mexco-tdxcel-dry-diamond-tile-drill/p41606

I've used the grab on slate sign. Just hold it in place for 2/3 minutes it'll cure within 24 hours provided weather temperatures are ok