Ah, the machine that nearly never was, from what I remember. Uncle Clive was apparently not interested in further upgrading the humble Spectrum after the Speccy Plus because he was putting his efforts into the ZX83, the machine that became the QL. It seems that the company Sinclair used in Spain, Investronica, WAS interested however, so they did.
The machine they produced, hardware wise, wasn't instantly recognisable as a Spectrum though. The new machine had 2 banks of 64K RAM to produce 128K, the good ol' Yamaha AY-3-8912 sound chip, an RS232 output and oddest of all, a numeric keypad that plugged into the front of the machine. Cooling was obviously an issue - Investronica got round this by putting a bloody great big metal heatsink down the right-hand side of the case, leading to the machine becoming known as the 'Toastrack' edition :)
Software wise there were 2 modes - 48K mode for full compatibility with older Speccy titles and native 128K mode. Native mode gave access to the improved BASIC, which had lost the keyword entry method of the older machine (no more pressing J "" to load a game :) and a proper screen editor for ease of programming. I can't remember much stuff being done in BASIC though, well, not at the time.
The machine launched in Spain first, because they'd produced it and because there were a lot of unsold Speccy Pluses over here. When it eventually got here it had lost the numeric keypad because of cost considerations - Sinclair was in dire financial trouble, which sadly resulted in not that many +128Ks being sold before he sold the whole lot, lock stock and barrel, to Amstrad in 1986. The machine that followed of course was the very CPC464-lookylikey Spectrum +2 which brought the Speccy back into the public eye.