[Retlaw]

I've had a reply from the experts, and the answer is pasted below. Another of the experts dates the photo around the 1880's. The tunic is red with black cuffs & the fancy bits are a very pale blue.
Retlaw.
He is as I suspected a Royal Engineer with 9 button tunic and the standard lacing around collar and an Austrian knot on the cuff. The collar lace appears slightly obscure but if you look at the left side and under his chin you can see that the lace surrounds the collar as normal for RE. Collar badges were not always worn until the late 1870s. The snake belt clasp was quite common on the 1870 Valise Equipment that was issued to RE, as well as infantry.