Newly adopted cat - Litter box problems

I recently adopted an adult cat from the RSPCA over the weekend. She's a sweetheart and has quickly settled in after only three days, crazy affectionate has some abandonment issues  and a gimpy leg (doesn't cause her any pain) but we'll tackle things one at time.

Only issue we've really had was waiting for her to use the litter box. We finally have and she chose to pee in the bath instead even though she has a choice of two litter boxes at two ends of the house. Just hoping this is a new home thing/marking her territory but if it's not how do I go about litter training an adult cat? Do I keep her out of the bathroom and hope she chooses litter boxes instead?  

Also should probably add she is strictly indoors due to being a carrier of catflu so do need to have her litter trained.

My first thought was more litter boxes. At least one more, maybe more, even if only on a temporary basis as close as possible to the places where she is peeing, then gradually move them till you have two in places that are acceptable to her. I would even put one int he bath for the moment, then after a few days, if she is using it take it out of the bath and put it on th efloor in the bathroom. I assume she has been vet checked? A urinary infection would cause her to avoid the litter box as a source of pain.

Okay cool, I'll look into picking more up tomorrow.

Yup, all she was checked before adoption/desexed/vaccinations so hopefully no issues there. 

If she was checked before you took her then she may not have had symptoms. A specific check for a UTI would not be part of adoption procedure - it requires a urine or blood sample. It might be worth another check.