Tom, it's better to use weight of honey as a measure, it takes some of the error out of guesstimations. Especially if you include total volume.
This may be a bit clovey, but luckily spice flavours will dissipate with time if you don't like it.
5lt honey = about 7.25kg
total volume = 17 litres, total weight = 19kg 19/17 = SG= 1.13
it's possible if you had a very low water content honey, that would could have an SG of 1.15, where many hydrometers drop the 1. from the front, which may be how you got 15. Take a photo of your hydrometer and upload it, it wont be hard for us to see how it works and help you out.
This is quite a heavy must, but CY17 will tolerate up to 15% so it will probably drop 115 points, leaving you with an FG around 1.035, which is very sweet, but not unpalatable if well balanced. I'm not sure how much acid you added, but you may need to add acid or tannin after.
It's generally a bad idea to add acid up front in meads, as honey is quite acidic already, and during fermentation the pH drops. This can cause your yeast stress and stuck ferments, so don't add any more until it's finished, and then I'd wait for it to age a bit. This is only an issue with mead. Many a wine maker will insist you add acid up front, ok with wine, not with mead.
Unfortunately I feel this will require some aging. The good thing is you made a fair whack, so you can take samplers along the way ;-)
Good luck, and keep us posted.