1 1/4" is no good for a 1 1/2" trap as the builders of our college found out when we moved in.
They fitted the smaller size to all the outlets, (over 75 outlets), and tried to seal them with excess silicone. They should have tested them afterwards because we discovered as soon as we started using them we had leaks everywhere. They had to come back at their expense and replace every single one of them plus pay for the damage the water had caused to the cupboards the sinks are fitted over.
Screwfix sell them.
Order online at Screwfix.com. Universal telescopic bottle trap with anti-vac, 76mm seal depth, dual 1¼" (32mm) and 1½" (40mm) inlet and outlet connections. Accepts pipe manufactured to BS 1451, BS 1455 and BS 1566. FREE next day delivery available, free collection in 1 minute.
www.screwfix.com
Order online at Screwfix.com. Chrome-plated brass body. Centre-pin dry-fixing connection for a quick and simple installation. Suitable for basins with or without overflow. Supplied with clicker waste, washers and plastic spacer for use with unslotted basins. FREE next day delivery available...
www.screwfix.com
Hope this helps.