Your photo looks reasonably like my Viola which blooms starting in June here on the northern edge of zone 5. However, this seems too early for a summer-blooming clematis like Viola. I would have expected a plant blooming at this time of year to be an early large-flowered plant.

We have had a very mild winter and some unseasonably warm weather here so maybe it is a bit early.

One thing you can do for this is go to Clematis on the Web and type in what you know to get a list of suggestions. There are many clematis that have Viol in the name and partial names work. So type in "Viol" under name, purple for color of tepal (combination of petals and sepals), yellow or cream (try both on different searches) for anther color, and May for bloom time. Leave everything else blank and hit search. You should get a list of possibilities. In deciding which, ignore the color in the photos since blues and purples usually don't photograph true to color and concentrate on petal (tepal) shape, size and amount of overlap or gap as well as any ruffles or twist and anther color to help you figure out what you have along with flower size (given in cm since this is a British site.) You can do the same with the other unknown you have posted. If you know what else you have that is blooming, that can help as well if you look up their bloom time to see if others you have are that far off of usual season. I still think an early large-flowered is more likely than that is actually Viola.