((Tx£1-£1)x58)+((-£1)x(100-58)) = £7.88

I think you may be missing brackets in the first section.. i think you could maybe interpret this a number of ways......

Maybe..

( 58(T£ - £1) ) + (-1£ x 42 ) = £7.88

58T£ - 58£ - 42£ = £7.88

58T£ - 100£ = £7.88

58T£ = £107.88

T£ = £107.88 / 58

T£ = 1.86