When cruising on the flat(ish) mid-ride, what gear are you in? More specifically, which chainring and roughly where on the cassette (bigger sprockets, middle sprockets or smaller sprockets)? The answer to that question should give you a good indication of whether 52/42 and your current cassette is optimal for you.
For example, if you are very comfortable riding in 42 x 17 or 19, then you probably either want to keep that combination on your bike or ensure that whatever chainset and cassette you replaced them with gave a similar gear with the chain somewhere in the middle of the cassette.
Conversely, if you routinely find yourself cross chaining on the flat - e.g. 42 x 23, 52 x 23 or 42 x 14/13 - then you should be considering what alternative chainring and/or cassette options will again give a similar gear with the chain somewhere in the middle of the cassette.
You can use the Ritzelrechner website to which Brucey has provide a link above to experiment with different chainring and/or cassette options and compare your current gearing with alternative options, in order to determine for yourself what combination is most likely to suit your needs and preferences.
The question of how best to get that extra bottom gear (or two) will depend somewhat on what you decide about what chainring(s) you decide you want for riding on the flat/shallow gradients. If you decide on a smaller outer/middle ring combination, then that will probably allow you to have a correspondingly smaller (or even more so) inner ring (with the front derailleur capacity likely to be the constraining factor, but possibly also the capacity of the rear derailleur). This is a somewhat iterative decision process: if you wanted to keep 52/42 chainrings, that would likely limit the reduction in size of the inner ring, and so you would need to consider alternative cassettes to get the extra lower gear(s).
As it happens, I have the same 13-26 cassette fitted to an old frame. I didn't want a wider cassette because I liked the small intervals between 17, 19, 21 and 23. I fitted a Spa RD2 triple
with 48/38/26 rings. I would have preferred the TD2 triple
with 46/36/24, but the frame required a braze on front mech and I doubted it would lower sufficiently for a 46 outer.
Currently your lowest two gears are 31" and 36". With a 26t inner they would be 27" and 31", and with 24t they would be 25" and 28". I can't see any reason to choose a 28t: fit the smallest you possibly can within the constraints of your derailleurs.