Lorraine i can well understand your fustration, my cat lexie who is much older than romeo, uses her litter box ritually, no problem, then oneday out of the blue she peed on our mat in the kitchen, and also pooped there, she has never done it again since, i also reached for advice from pet-talk, and i believe she was stressed by something, because shes fine now, we had the dishwasher on, and it was playing up at the time, and maybe it made this awful noise all night, so we think thats what did it, but i have since learned never leave towels around, she will pee on those too.
Apart from stress, or bladder problem, i cant help much, hope things improve for you and romeo starts using his toilet again, cheers