Reading Hammond and Hammond (1919) The Skilled labourer I came across the expression of £10% in the quoted primary sources, ie, at 34:
The Duke of Northumberland wrote to the H.O. on May 26 that work had been resumed-'the occupiers having made their own terms respectively with the refractory workmen, and thereby departed from the Union which they had recently established in their own defence; whilst the original and imposing Union of the Pitmen is in full authority and force. In many cases new covenants have been made upon fair and tenable principles, abrogating some hard and indefensible customs and giving an advance of wages upon an average of £10 per cent. In some cases a precipitate and absolute concession has been made to the demands of the pitmen more I apprehend in the eagerness of mercantile zeal than from any positive and impending intimidation' (H.O., 52. 14).
Given that mining wages were expressed as 30/- a fortnight, or £2/6/-, how am I to read a £10% advance on a yearly bond for £65 pounds. Naturally I'd read it, as a modern, as an on the spot payment of £6/6/- (or six guineas) to be deducted from future wages. Further reading indicates that 10% advance probably means 10% increase in the language of the day, rather than a sign-on advance of wages. The £% is throwing me though, is it an indicator that the per cent. is expressed in money?
I've tried googling this, but £X% throws results as if for X%.
Having been motivated by this question to do more research, I perused the OED for per centum and related entries. There it is made clear that the use of the (pseudo-Latin legalese) phrase per centum (or it's abbreviation per cent) is meant to be translated literally, as per hundred.
Therefore the phrase inquired upon, £10 per cent, should be read as £10 per hundred [pounds].
To me it simply looks like it is meant to say "10 pounds sterling per 100".
"cent" is short for the Latin word for hundred, "centum", and £ is the symbol for "pounds sterling".