Its first offer, worth pounds 1.56bn when it was launched last month, has already stretched the group and it will struggle to lift its new bid much above the pounds 1.6bn counter from Southern Electric.
The SE offer is structured to be worth 1,013p a Southern Water share. The talk is that ScottishPower may go to 1,020p, possibly 1,030p.
The Scottish group's shares have fallen from 338p since it produced its bid. They touched 304p on Tuesday, rallying 6p to 311p yesterday.
SE was a shade firmer at 674p and Southern Water was little changed at 987p.
The rest of the stock market was - almost - lost in a haze of summer sunshine and sporting attractions.
Trading was again at a low ebb with the appeal of Royal Ascot and the lingering impact of England's Euro `96 Cup win proving decisive influences.
There was, however, enough energy left for the market to brush aside Whitehall's flat sales statistics for last month. There was a tendency to pay more attention to the recent upbeat retail survey from the Confederation of British Industry and the shop floor evidence from various retailing groups.
The May retail sales also prompted thoughts that the Chancellor could be tempted into another interest rate cut, a possibility which helped sentiment.
Even so the market could not disguise its lacklustre attitude which left the FT-SE 100 index down 3.2 points at 3,753.2.
BTR fell 3p to 256p in busy trading as the market prepared for negative analyst meetings next week.
Granada was the best-performing blue chip, gaining 20p to 869p with Kleinwort Benson providing much of the impetus. In a bulletin on the leisure industry - A Golden Age for Hospitality - analyst Paul Slatttery suggested Granada could hit 900p if it completes its Forte disposal programme and continues to achieve expected growth targets.
The arrival of Abu Dhabi's investment arm at Manchester Utd with a 4 per cent shareholding helped the football club to recover from the impact of last week's directors' share sales with the price moving 18p to 459p.
British Biotech had an eventful session with the sale of nearly 1.2 million shares at 2,427p sending the price tumbling 27p.
But towards the close an array of buyers signalled their willingness to take on shares and the price ended 30p up at 2,620p.
Cable & Wireless gained 7.5p to 427.5p on its pounds 60m Asia Satellite windfall and ABN Amro Hoare Govett support. Kingfisher enjoyed a push from UBS, improving 9p to 639p. Greenalls, the pub group about to be ousted from Footsie, rose 11.5p to 590p on SBC Warburg support.
Albert Fisher, the food group, held at 47p. Rumours that chairman Stephen Walls was to step down into a non-executive role were denied by the company.
Charter, selling its 65.3 per cent holding in Cape, the industrial group, fell 34p to 916p. Rutland Trust, the entrepreneurial group,is acquiring 25 per cent of Cape at 130p a share with most of the rest of Charter's stake bound for institutions. Cape was firm at 166p and Rutland held at 54p.
Johnson Matthey, the metals group, slipped 2p to 628p with Warburg thought to have some of the Minorco stake still on its books.
ADT, the Bermuda-based security group run by Michael Ashcroft, eased 23p to 1,195p as it produced an pounds 84.9m share exchange offer for Automated Security, up 8p at 39p.
Elys, the Wimbledon department store where a furious bid battles rages, stayed at 690p as Morley's Stores, an unquoted London department store chain, lifted its bid to 700p, topping an offer from Panther Securities by 15p.
Cantors, the furniture retailer, was suspended at 175p, up 6p. It is in talks with Harveys, a chain of home furnishing shops, controlled by the family of Lord Harris, of Carpetright fame.
Two constituents of AIM marred the junior market's first anniversary celebrations. Creos, supplying generators for medical imaging, said it had lost a contract and half-time losses increased to pounds 998,000. The shares fell 17p to 72p; they were 137p last year. Memory Corporation, hit by the fall in chip prices, lost another 10p to 85p, against 553p last year.
Dana Petroleum, with exploration interests in the former Soviet Union, put on 0.5p to 10.5p; the shares were 7p last week. Rumours of a strike are going the rounds.
r AIM continues to attract recruits. IES, on the verge of graduating from Ofex earlier this year, is due to arrive today.
The shares closed on Ofex at 290p with the warrants at 275p. The security equipment group made a profit of pounds 616,000 last year. A pounds 275,000 loss in the first half of its current year is blamed on development costs and a tendency for income to be delayed until the second six months.
r Birkdale, a small advertising and marketing group, held at 6.75p. The company has cut bank borrowings by selling a property for pounds 750,000 and is considering switching loans of pounds 950,000 from chairman Kevin Morley into convertible preference shares. Mr Morley is a former managing director of Rover cars.