=query({'Sheet1'!I2:I,'Sheet1'!L2:L;'Sheet2'!I2:I,'Sheet2'!L2:L},"Select Col1, sum(Col2) group by Col1 label Col1 'Month'")

| Month       | sum
|
| 2018/12     | £ 35.00
| 2019/01     | £ 155.00
| 2019/02     | £ 60.00
| 2019/03     | £ 210.00
| Grand Total | £ 460.00

| Month       | sum       | Sheet1  | Sheet2
|
| 2018/12     | £ 35.00   | £10     | £25
| 2019/01     | £ 155.00  | £100    | £55
| 2019/02     | £ 60.00   | £30     | £30
| 2019/03     | £ 210.00  | £110    | £100
| Grand Total | £ 460.00

={QUERY({Sheet1!I2:I, Sheet1!L2:L; Sheet2!I2:I, Sheet2!L2:L},
  "select Col1, sum(Col2)
   where Col1 is not null
   group by Col1
   label Col1 'Month', sum(Col2)'Sum'", 0),
 {"Sheet1"; QUERY(ARRAYFORMULA(IFERROR(VLOOKUP(QUERY(QUERY({Sheet1!I2:I, Sheet1!L2:L},
                                                     "select Col1, sum(Col2)
                                                      where Col1 is not null
                                                      group by Col1", 0),
                                               "select Col1", 0),
                                       {Sheet1!I2:I, Sheet1!L2:L}, 2), )),
            "select Col1 where Col1 is not null", 0)},
 {"Sheet2"; QUERY(ARRAYFORMULA(IFERROR(VLOOKUP(QUERY(QUERY({Sheet2!I2:I, Sheet2!L2:L},
                                                     "select Col1, sum(Col2)
                                                      where Col1 is not null
                                                      group by Col1", 0),
                                               "select Col1", 0),
                                       {Sheet2!I2:I, Sheet2!L2:L}, 2), )),
            "select Col1 where Col1 is not null", 0)}}