Fitting a SP2B to a 28mm copper pipe is uncommon as most homes have 22mm copper, however, if you need to fit to 28mm here is how do it:-

28mm pipework next to cylinder

The fittings you need are as follows:-
28mm Straight Compression Couplings (retail price £3.88 each)
28mm to 22mm reducers (retail price £2.15 each)
28mm pipe slice
22mm pipe slice
short length of 22mm pipe.

28mm fitting materials

Fitting Sequence
Do as much of this you can on a work bench first (do not hold white motor barrel)
Cut a 50mm long slice of 22mm pipe
Replace the 28mm olive on one side of the 28mm compression joint and fit the 28mm to 22mm reducer.
Insert the 22mm pipe and tighten the compression joint to compress the reducer.
Insert the exposed 22mm pipe into the SP2B and tighten the SPB compression join.

Repeat for the other side of the fitting.
If you need to double boost add an extra 50mm of copper pipe to join your SP2B and SP1U together.

Offer up the completed assembly to your existing pipe to know how much 28mm pipe you need to cut out.
Cut out the length of 28mm pipe.
Insert SPB assembly and tighten joints to 28mm compression joints.