This section is from the book "Real Property, An Introductory Explanation Of The Law Relating To Land", by Alfred F Topham. Also available from Amazon: The New Law Of Property.
When two or more pieces of land are mortgaged for separate amounts by separate mortgages, the borrower is now entitled to redeem each mortgage separately, unless
(1) both pieces of land belong to the same borrower, and are held in mortgage by the same mortgagee, and
(2) one at least of the mortgages was made before the 31st of December, 1881, or contained a, clause allowing the mortgagee to "consolidate."
History of the rule.
At Common Law. - If the same borrower mortgaged two or more pieces of land to the same mortgagee, the mortgagee could "consolidate" the mortgages and treat them as one mortgage for all the advances together.
Thus, Fitzgerald mortgaged a freehold house, called " Wood-side " (worth £1000), to Moses for £500 (as in the form on p. 224). Later, Fitzgerald mortgaged a brickfield to Moses for £1000. The brickfield became depreciated and is now worth only £500. Fitzgerald would be glad to clear off the £500 from Woodside, but he would not be so anxious to pay off the £1000 from the brickfield. On the other hand, Moses would be very anxious to get the £1000 paid off as the security is insufficient; but he would not necessarily wish to have the £500 paid off. The rule was that if Fitzgerald claimed to pay off the £500 and redeem "Woodside, Moses could refuse to allow him to do so, unless he also paid off the £1000 on the brickfield. That is, he could consolidate the two mortgages and treat them as one mortgage for £1500 on both the properties.
By the Conveyancing Act, 1881, s. 17, when a mortgage is made after 1881, the mortgagee has no power of consolidation unless such a power is expressly given in the mortgage deeds or one of them.
A clause expressly allowing consolidation is now frequently inserted in mortgage deeds; consequently the common law on this subject is still most important.
If either of the mortgages excludes s. 17 of the Conveyancing Act, the old law applies.
Further rules when consolidation is allowed. - Consolidation may apply even where the mortgages were made by different mortgagors to different mortgagees if at any one time the equity of redemption in all the mortgages becomes vested in the same person, and the mortgages all become vested in the same mortgagee.
Pledge v. White (1896), A. C. 187.
In 1863 James Banks mortgaged • 5 Shakespeare Terrace and one other property to John Banks for £500 each. In 1865 James Banks mortgaged one other property to Hobday and four other properties to other mortgagees.
Between 1871 and 1873 all the mortgages except one were transferred to Brockman.
Brockman died in 1877, and in 1890 the remaining mortgage was transferred to Brockman's executors, who thus became mortgagees of all seven properties. Pledge (who had acquired the equity of redemption of all the properties from John Banks) claimed that he was entitled to redeem 5 Shakespeare Terrace by paying off the £500 secured on that property only.
Held, Brockman's executors can claim to have all the mortgages paid off.
That is to say, if at any one time there is the same mortgagor and the same mortgagee for all the properties, the right of consolidation may arise.
The difference between tacking and consolidation is as follows: -
In the case of tacking, one piece of land is mortgaged by the same person to different mortgagees.
In the case of consolidation, several pieces of land are mortgaged by the same person to the same mortgagee.
Consolidation as against other mortgagees.
When the borrower has mortgaged one of the pieces of land several times to different mortgagees, and makes a mortgage of another piece of land to one of the same mortgagees, the position becomes rather complicated; the rule is then as follows: -
When a mortgagee obtains a right of consolidation, any persons who afterwards take mortgages of either piece of land, take subject to his right of consolidation.
But any persons who took second mortgages before the first mortgagee obtained his right to consolidate, cannot be prejudiced by the consolidation.
Hughes v. Britannia Building Society,  2 Ch. 607.
In 1894, Worley mortgaged No. 11 Manchester Avenue to the Society for £87.
In 1895 Worley mortgaged "Rocklea " to the Society for £900.
The next day Worley mortgaged "Rocklea " to Hughes for £7000.
In 1897 Worley mortgaged No. 9, Manchester Avenue to the Society for £687.
In 1905 Rocklea was sold.
The Society claimed that after payment to them of the £900 due on the mortgage of "Rocklea," they were entitled to claim that the mortgages for £87 on No. 11, and for £687 on No. 9 should all be paid before Hughes got anything.
Held, the society are entitled to consolidate the mortgage on No. 11 as it was made before the mortgage to Hughes.
But they are not entitled to consolidate the mortgage on No. 9 as it was made after the mortgage to Hughes.
Therefore the society could claim £987 out of the purchase money, and the rest must go towards paying the £7000 to Hughes.