Note that male Scarlet in basic plumage indeed does look like females, but they keep their black wings & tail, making ID of ad. male Scarlet in basic plumage rather straight forward:
"Basic" male Summer (of which I've observed many) is similar to the males many have observe in the US except a bit paler - as I think others already established in previous posts. This leaves us with four possibilities: A juv. of either species or a female of either species. A loudly singing juvenile? Probably not. Furthermore, there is the edging on the wing coverts: All info I have been able to locate indicate that Scarlet only has it infrequently and that it is rather indistinct if present. On the other hand the edging to the wing coverts seem to be a pretty standard thing in both juv. and female Summer, though certainly not present in all individuals. Regardless, the colours of the underparts alone (as noted by other in previous posts) would be enough for me to think Summer. Bill shape & colour point in the same direction, though bill colour only can be regarded as an indication of ID and the shape clearly is somewhat difficult to use from this angle.